Data Structure and Algorithms Problems
Current Status  Stats 

Total C++ Problems  188 
Total Python Problems  15 
Current Daily Streak  11 
Last Streak  06/20/2019  06/21/2019 
Current Streak  06/23/2019  07/03/2019 
Note: Some of the code here is old and was written when I was learning C++. It might be possible that code is not safe or making wrong assumptions. Please use with caution. Pull requests are always welcome.
LinkedList Problems
Problem  Solution 

Find the nth node of linked list from last.  nthToLastNode.cpp, nth_to_last_node.py 
Add numbers where each digit of the number is represented by node of a linkedlist. Give output as a linked list.  add_two_numbers_lists.cpp, add_two_numbers_list.py 
Swap nodes of a linkedlist without swapping data.  swapNodesWithoutSwappingData.cpp, swap_nodes_without_swapping_data.py 
Reverse a linked list, iteratively and recursively  reverseLinkedListIterAndRecurse.cpp, reverse_linkedlist.py 
Given a linked list, reverse alternate nodes and append at the end.  reverseAlternateNodes.cpp 
Only given a node pointer, delete the node from the linked list.  deleteNode.cpp 
Delete the entire linkedlist.  deleteLinkedlist.cpp 
Print middle node of linkedlist without iterating twice.  printMiddleNode.cpp 
Determine if a linked list is a pallindrome.  listPallindrome.cpp 
Insert data in a sorted linked list.  insertInASortedLinkedList.cpp 
Determine the intersection(merging) point of two given linked list.  findIntersectionPointOfLists.cpp, intersection_of_lists.py 
Clone a linkedlist which has next and an random pointer, Space Complexity  O(1).  cloneListWithRandomPtr.cpp, clone_list_with_random_ptr.py 
Given a sorted linked list with duplicates, remove duplicates in one iteration.  removeDuplicatesFromSortedList.cpp 
Using Floyd's cycle finding algorithm, detect if a linkedlist contain cycle, if it does contain cycle, remove the loop  floyedCycleDetection.cpp 
Sort a linked list using merge sort  merge_sort.cpp 
Given a singly linked list L_{0} > L_{1} > … > L_{n1} > L_{n}. Rearrange the nodes in the list (in place) so that the new formed list is : L_{0} > L_{n} > L_{1} > L_{n1} > L_{2} > L_{n2 ....}  rearrange_list.cpp 
Include
Include contains single header implementation of data structures and some algorithms.
Data Structure/Algorithm  Implementation 

Generic Macros and Algorithms like swap, random number generation  generic.h 
Generic Stack Implementation  stack.h 
Generic Queue Implementation  queue.h 
Generic List Implementation  list.h 
Binary Search Tree Implementation  binarySearchTree.h 
Quick Sort Implementation  quickSort.h 
Merge Sort Implementation  mergeSort.h 
Selection Sort Implementation  selectionSort.h 
Bubble Sort Implementation  bubbleSort.h 
Linux Kernel Double LinkedList Implementation  double_linked_list.h 
Generic Graph Implementation (Adjacency List)  graph.h 
Heap Sort Implementation  heap_sort.h 
My own string library implementation  pstring.h pstring.cpp 
Bit Manipulation Problems
Problem  Solution 

Determine if a number is a power of 2.  power_of_2.cpp 
Add two binary number represented as string.  addBin.cpp 
Determine the next power of 2 for a given number.  next_power_of_2.cpp 
Using bit manipulation determine if a number is multiple of 3.  multiple_of_3.cpp 
Determine endianess of the machine, print a number in reverse Endianess.  reverseEndianness.cpp 
Find the parity of given number.  find_parity.cpp 
Implement fast multiplication of a number to 7 using bit manipulation.  multiply_by_7.cpp 
Reverse bits of unsigned integer (two methods  Reversing bit by bit & divide and conquer).  reverseBitsOfAnInteger.cpp 
Small function to determine position of right most set bit in a given integer.  right_most_set_bit.cpp 
Given a vector of numbers, only one number occurs odd number of times, find the number.  find_odd_one_out.cpp 
Given two integers, determine if their sum would be interger overflow.  integerOverflow.cpp 
How many bit flip operation would require to convert number A to B.  countNumberOfBitFlips.cpp 
Given a number x and two positions (from right side) in binary representation of x, write a function that swaps n right bits at given two positions and returns the result. It is also given that the two sets of bits do not overlap.  swapSetOfBits.cpp 
Add two numbers without using any arithmetic operators  addition_without_operators.cpp 
Louise and Richard play a game. They have a counter set to N. Louise gets the first turn and the turns alternate thereafter. In the game, they perform the following operations:

counter_game.cpp 
Determine if two integers are of opposite signs.  check_opposite_signs.cpp 
Swap two bits at position p and q of a given integer.  swapBits.cpp 
Check if a number is power of 4.  check_if_power_of_4.cpp 
Cracking the coding interview problems
Problem  Solution 

Problem 11 : Edition 6: Write an algorithm to determine whether a string has unique characters or not. Can we do it without using additional data structures?  11hasUniqueChars.cpp, 11hasUniqueChars.py 
Problem 12 : Edition 5: Reverse a string when you are a pass a null terminated C string.  12edi5reverseString.cpp 
Problem 12 : Edition 6: Given two strings, determine if one is permutation of other.  12permstrings.cpp, 12permstrings.py 
Problem 13 : Edition 5: Write an algorithm to remove duplicate chars from a string.  13edi5removeDuplicates.cpp 
Problem 13 : Edition 6: URLify: Replace all the spaces in a string with '%20'. Preferebly Inplace  13URLify.cpp 
Problem 14 : Edition 6: Given a string, write a function to check if it is a permutation of a pallindrome.  14pallindromepermutations.cpp 
Problem 15 : Edition 6: There are three possible edits that can be performed on a string  Insert a char, Delete a char, Replace a char. Given two strings, determine if they are one or 0 edit away.  15oneeditaway.cpp 
Problem 16: Implement a method to perform basic string compression. Example string aabcccccaaa should be compressed to a2b1c5a3, however if compressed string is bigger than original string, return original string  16stringcompression.cpp 
Problem 17: Rotate the matrix clockwise( & anticlockwise) by 90 degrees  17matrixrotation.cpp 
Problem 18: Write an algorithm such that if an element of MxN matrix is 0, its entire row and column is set to 0.  18zeromatrix.cpp 
Problem 19: Given two strings s1 and s2, determine s2 is rotation of s1 using only one call to a function which checks whether one string is rotation of another.  19stringrotation.cpp 
Problem 21: Remove duplicates from an unsorted linked list. What if no temporary buffer is allowed.  21removedups.cpp 
Problem 22: Determine k^{th} node from the last of a singly linked list. (Iterative and Recursive Approaches)  22kthToLast.cpp 
Problem 23: Implement an algorithm to delete a node in the middle of a singly linked list  23deletemiddlenode.cpp 
Problem 24: Partition a linked list around a value x, all the nodes smaller than x come before all the nodes greater than equal to x  24partition.cpp 
Problem 25: You have two numberse represented by a linked list where each node contains a single digit. The digits are stored in reversed order, such that 1's digits are at the head of the list. Write a function that adds the two numbers and returns the sum as a linked list.Example:

25addlists.cpp 
Problem 26: Determine if linked list is palindrome( 2 iterative and one recursive approach  26palindrome.cpp 
Problem 27: Determine if two singly linked list intersect, if yes, return the intersecting node. The intersection is defined based on reference not on values  27intersection.cpp 
Problem 28: Detect if the linked list have a loop, Find the start node of the loop and remove the loop  28loopdetection.cpp 
Dynamic Programming Problems
Problem  Solution 

N^{th} Fibonacci term using different memoization techniques  fibonacci.cpp 
Longest Common Subsequence Problem  lcs.cpp, longest_common_subsequence.py 
Maximum Value Contigous Subsequence Problem wiki  max_subsequence.cpp 
Catalan number  Count the number of possible Binary Search Trees with n keys  catalan_number.cpp 
Calculate the number of unique paths from source origin (0, 0) to destination (m1, n1) in a m x n grid. You can only move either in down or right direction.  unique_paths.cpp 
01 Knapsack Problem: Imagine you are a thief and you want to steal things with room full of things. You have a knapsack which can handle maximum capacity of weight W, and you want to fill it up such that it's worth is maximum. Being an intelligent thief, you know weights and values of each item in room. How would you fill your knapsack, such that you get the maximum possible value, such that you can only fill upto capacity W.  0_1_knapsack_problem.cpp 
Tree Problems
Problem  Solution 

Iterative Level order traversal of Tree using queue  levelOrderTraversalIterative.cpp, level_order_tree_traversal_iterative.py 
Recursive Level order traveral of Tree  levelOrderTraversalRecursive.cpp, level_order_tree_traversal_recursive.py 
ZigZag Traversal of Tree  zigZagTraversal.cpp, zig_zag_traversal.py 
Predecessor and Successor of a given node in Binary Search Tree  predecessorSuccessor.cpp 
Given values of two nodes in a Binary Search Tree, find the Lowest Common Ancestor (LCA). Assume that both the values exist in the tree.  lowestcommonancestor.cpp, lowest_common_ancestor.py 
Given a binary tree (unlike binary search tree), find the Lowest Common Ancestor (LCA).  lowestcommonancestorbinarytree.cpp 
Given a binary tree, print out all of its roottoleaf paths one per line.  printAllRootToLeafPath.cpp 
Determine if a tree is sum tree. A SumTree is a Binary Tree where the value of a node is equal to sum of the nodes present in its left subtree and right subtree. An empty tree is SumTree and sum of an empty tree can be considered as 0. A leaf node is also considered as SumTree.  sumTree.cpp 
Convert a tree to sumTree, such that each node is sum of left and right subtree of the original tree.  convert_to_sum_tree.cpp, convert_to_sum_tree.py 
Convert a sorted array to balanced binary search tree.  sortedArrayToBST.cpp 
Given a binary tree, generate sum of each vertical column.  verticalSum.cpp 
Given a binary tree and key, node with key exists in tree. Find all the ancestors of the node with key, ancestor here are the nodes which are in straight path from node to root.  node_ancestors_in_root_path.cpp 
Given a binary tree and key, return the level of the node with key. Root is at level 1, and if node with key does not exists in tree, return 0  level_of_node.cpp 
Given a binary tree, find all the paths from root to nodes, whose sum is k.  k_sum_paths.cpp 
Given a binary tree, print its nodes level by level in reverse order. i.e. all nodes present at last level should be printed first followed by nodes of secondlast level and so on.. All nodes for any level should be printed from left to right.  reverseLevelOrderTraversal.cpp 
Invert a binary tree, recursively and iteratively.  invert_a_tree.cpp 
Given a Binary Search Tree, find ceil and floor of a given key in it. If the given key lie in the BST, then both floor and ceil is equal to that key, else ceil is equal to next greater key (if any) in the BST and floor is equal to previous greater key (if any) in the BST  floor_ceil_bst.cpp 
Find kth smallest element in a binary search tree  kth_smallest.cpp 
Validate if a given binary tree is a binary search tree.  validate_bst.cpp 
Given a Binary Search Tree and a target number, return true if there exist two elements in the BST such that their sum is equal to the given target.  find_target_k.cpp 
Given a nonempty binary search tree and a target value, find the value in the BST that is closest to the target. Also, to note that the target value is a floating point. There will be only one unique value which is closest to the target.  closest_bst_value.cpp, closest_bst_value.py 
Given a binary tree, traversing preorder, construct a string output containing node values and parenthesis. The null node needs to be represented by empty parenthesis pair "()". And you need to omit all the empty parenthesis pairs that don't affect the onetoone mapping relationship between the string and the original binary tree. Examples in code file  string_from_tree.cpp 
String Problems
Problem  Solution 

Implementation of RobinKarp algorithm for string search  robinKarpStringMatching.cpp 
Find next permutation of a given string, ie. rearrange the given string sucht a way that is next lexicographically greater string than given string  next_permutation.cpp 
Implementation of Z algorithm for pattern matching  z.cpp 
Test cases for self created string library  pstring_test.cpp 
Get the length of the last word in a string.  length_of_last_word.cpp 
Find the difference between two string. String t is generated by random shuffling string s and then add one more letter at a random position. Determine the character which is different in t  find_difference.cpp 
Common Data Structure and logic problems
Problem  Solution 

Print the contents of matrix in a spiral order  matrix_spiral_print.cpp 
Given a M x N matrix, rotate it by R rotations anticlockwise, and show the resulting matrix.  rotate_matrix.cpp 
Rotate an array by r elements ( left or right )  array_rotation.cpp 
Given an array of repeating/nonrepeating intergeres, determine the first nonrepeating int in this array  first_non_repeating_int.cpp 
In Quantumland, there are n cities numbered from 1 to n. Here, c_{i} denotes the i^{th} city. There are n−1 roads in Quantumland. Here, c_{i} and c_{i+1} have a bidirectional road between them for each i < n.There is a rumor that Flatland is going to attack Quantumland, and the queen wants to keep her land safe. The road between c_{i} and c_{i+1} is safe if there is a guard in c_{i} or c_{i+1}. The queen has already placed a few guards in some of the cities, but she is not sure if they are enough to keep the roads safe. She wants to know the minimum number of new guards she needs to hire. See comments in solution for input/output details.  save_quantamland.cpp 
You are given an integer N. Find the digits in this number that exactly divide N (division that leaves 0 as remainder) and display their count. For N=24, there are 2 digits (2 & 4). Both of these digits exactly divide 24. So our answer is 2. See more details in header comment of the solution file.  findDigits.cpp 
Encrypt and then decrypts a text using Caeser Cipher.  caeser_cipher.cpp 
Encrypt and then decrypts a text using Vigenère cipher.  vigenere_cipher.cpp 
Generate binary numbers between 1 to N efficiently.  n_binary.cpp 
Implement power function  power_function.cpp 
Math Problems
Problem  Solution 

Print all the permutations of a string. Example: Permutations of ABC are ABC, ACB, BCA, BAC, CAB, CBA  string_permutations.cpp 
Euclidean algorithm to find greatest common divisor of two numbers. (Iterative and recursive)  gcd.cpp 
Implement pow(x,y) using divide and conquer approach. Try implementing it in O(logn)  pow.cpp 
Calculate factorial of large number, say 100 (it will have 158 digits)  factorial_of_large_num.cpp 
Generate all possible words from a number entered on a traditional mobile keypad  phone_digits.cpp 
Given a string representation of a number, remove n characters from the string such that number representation is lowest possible.  lowest_possible_number.cpp 
Detect if a number is a happy number. A number is happy number if sequence of operations where number is replaced by sum of square of its digits leads eventually to 1. A number is not a happy number if we are in an infinite loop when above operations are performed.  happy_number.cpp 
Stack Problems
Problem  Solution 

We have series of n daily price quotes for a stock. We need to calculate span of stock's price for all n days. Span for ith day is defined as maximum number of consecutive days, for which the price of the stock was less than or equal to ith day. For stock quotes {100, 60, 70, 65, 80, 85} span will be {1, 1, 2, 1, 4, 5}. Span for day 1 is always 1, now for day 2 stock is at 60, and there is no day befor it when stock was less than 60. So span remains 1. For day 3, the stock is priced at 70, so its span is 2, as previous day it was 60, and so on.  stock_span_problem.cpp 
Given an infix expression, convert it to postfix expression, Example (A+B)*C > AB+C*  infix_to_postfix.cpp 
Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.The brackets must close in the correct order, "()" and "()[]{}" are all valid but "(]" and "([)]" are not.  valid_parenthesis.cpp 
Sort and Search Problems
Problem  Solution 

Given a sorted vector, return first index of the occurrence of a value in vector, if number does not exist, return 1  first_occurrence_binary_search.cpp 
Find the first repeating element in an array of integers. Given an array of integers, find the first repeating element in it. We need to find the element that occurs more than once and whose index of first occurrence is smallest.  firstRepeatingElement.cpp 
Given a list of unsorted integers, A={a_{1},a_{2},…,a_{N}}, Find the pair of elements that have the smallest absolute difference between them? If there are multiple pairs, find them all.  closest_numbers.cpp 
Given a sorted array, determine index of fixed point in this array. If array does not have a fixed point return 1. An array has a fixed point when index of the element is same as index i.e. i == arr[i], Expected time complexity O(logn)  fixedPoint.cpp 
Find the maximum element in an array which is first increasing and then decreasing. Input: arr[] = {8, 10, 20, 80, 100, 200, 400, 500, 3, 2, 1}, output : 500. Array may be strictly increasing or decreasing as well. ExpectedTime complexity is O(logn).  findMaximum.cpp 
Given an array of positive and/or negative integers, find a pair in the array whose sum is closest to 0.  findClosestPairToZero.cpp 
Numeros, the Artist, had two lists A and B, such that B was a permutation of A. Numeros was very proud of these lists. Unfortunately, while transporting them from one exhibition to another, some numbers were left out of A. Can you find the missing numbers? Notes:

missingNumbers.cpp 
Find the closest pair from two sorted arrays. Given two sorted arrays and a number x, find the pair whose sum is closest to x and the pair has an element from each array. We are given two arrays ar1[0…m1] and ar2[0..n1] and a number x, we need to find the pair ar1[i] + ar2[j] such that absolute value of (ar1[i] + ar2[j] – x) is minimum.  closestPairSorted.cpp 
Given an array A of n elements, find three indices i, j and k such that A[i]^2 + A[j]^2 = A[K]^2. O(n2) time complexity and O(1) space complexity  squareSum.cpp 
Given an unsorted array arr[0..n1] of size n, find the minimum length subarray arr[s..e] such that sorting this subarray makes the whole array sorted.  minLengthUnsortedArray.cpp 
Find the missing number in Arithmetic Progression  missingNumber2.cpp 
Find the common elements in 3 sorted vectors  commonIn3Arrays.cpp 
Find all the pairs with a given sum in an unsorted array/vector  find_pairs_with_sum.cpp 
Given an array, find peak element in it. A peak element is an element that is greater than its neighbors.  peak_element.cpp 
Given a sorted array of distinct nonnegative integers, find smallest missing element in it.  smallest_missing.cpp 
Move all zeros in the vector to the end  move_zeros.cpp 
Graph Problems
Problem  Solution 

Depth First Traversal of a Graph  dfsDemo.cpp 
Breadth First Traversal of a Graph  bfsDemo.cpp 
calculate the shortest distance from the start position (Node S) to all of the other nodes in the graph using Dijkstra algorithm.  dijkstrashortestreach.cpp 
Calculate total weight of Minimum Spanning Tree of a given graph ( sum of weights of edges which forms MST) using Prim's algorithm  primsMST.cpp 
Print Minimum Spanning Tree( MST ) of a given graph using Kruskal's algorithm.  kruskalMST.cpp 
Create a program to generate a Huffman encoding for each character as a table.  huffman_encoding.cpp 
Search a given word in a 2D board containing letters. The word can be constructed by sequentially traversing adjacent horizontal or vertical cells. In a sequence to form word, letter on same position can not be used more than once. (Check top of file for examples.)  grid_word_search.cpp 
Given a 2D screen, location of the pixel and new value of the color to fill, replace the color of the pixel and all the adjacent(up, below, left, right) same colored pixel with new color. This is same as flood fill (remember the bucket symbol) a region in MSPAINT.  flood_fill.cpp 
Greedy Problems
Problem  Solution 

Given two integer arrays, A and B, each containing N integers. You are free to permute the order of the elements in the arrays. Is there an permutation A', B' possible of A and B, such that, A'_{i}+B'_{i} ≥ K for all i, where A'_{i} denotes the i^{th} element in the array A' and B'_{i} denotes i^{th} element in the array B'.  two_arrays.cpp 
John is taking orders. The i^{th} order is placed by the i^{th} customer at t_{i} time and it takes d_{i} time to procees. What is the order in which the customers will get their orders? (see more details in solutions's comments)  orders_order.cpp 
Backtracking Problems
Problem  Solution 

You are given a digit string (e.g "1234", "567" etc), provide all possible letter combinations we could generate from this digit string, based on the mapping we see on the telphone/mobile dialpad. If you have typed SMS in old style phones, you would know. For e.g. "1" is mapped to "abc", 2 is mapped to "def". You can refer to the image..

dialpad_combinations.cpp 
Implement wildcard pattern maching with support for '?' & ''.

wild_card_matching.cpp 
Given a 2D board and list of words from a dictionary, find all the possible words on board fromt the list. (Check example in the solution)  word_search.cpp 
Leet code Problems
Problem  Solution 

Given a sorted integer array without duplicates, return the summary of its ranges. For example, given [0,1,2,4,5,7], return ["0>2","4>5","7"].  summary_ranges.cpp 
Given a 2D matrix, with following properties

search2DII.cpp 
Given an unsorted integer array, find the first missing positive integer.Example: [1,2,0] should return 3 and [3,4,1,1] should return 2. Expected time complexity O(n) and solution should use constant space  firstMissingPositiveNum.cpp 
Given an unsorted array of integers, find the length of the longest consecutive elements sequence. For example: Given [100, 4, 200, 1, 3, 2]. The longest consecutive elements sequence is [1, 2, 3, 4]. Return its length: 4. Algorithm should run in O(n) complexity.  longestConsecutiveSeq.cpp 
Given two sorted integer arrays nums1 and nums2, merge nums2 into nums1 as one sorted array.You may assume that nums1 has enough space (size that is greater or equal to m + n) to hold additional elements from nums2. The number of elements initialized in nums1 and nums2 are m and n respectively.  mergeArrays.cpp 
Given an array of nonnegative integers, you are initially positioned at the first index of the array. Each element in the array represents your maximum jump length at that position. Determine if you are able to reach the last index. For example:

jumpGame.cpp 
Given a positive integer, return its corresponding column title as appear in an Excel sheet. For example 1 > A, 2 > B,...26 > Z, 27 > AA, 28 > AB, ...705 > AAC  excelColSheetTitle.cpp 
Given an array nums, write a function to move all 0's to the end of it while maintaining the relative order of the nonzero elements. For example, given nums = [0, 1, 0, 3, 12], after calling your function, nums should be [1, 3, 12, 0, 0].  moveZeroes.cpp 
Given an array of integers, find if the array contains any duplicates. Function should return true if any value appears at least twice in the array, and it should return false if every element is distinct.  containsDuplicate.cpp 
Given a list, rotate the list to the right by k places, where k is nonnegative. For example:

rotateList.cpp 
Given two words word1 and word2, find the minimum number of steps required to convert word1 to word2. (each operation is counted as 1 step.). You have the following 3 operations permitted on a word:

editDistance.cpp 
Given a binary tree, Populate each next pointer to point to its next right node. If there is no next right node, the next pointer should be set to NULL. Initially, all next pointers are set to NULL.You may only use constant extra space.You may assume that it is a perfect binary tree (ie, all leaves are at the same level, and every parent has two children).  connectNextPointers.cpp 
Given n pairs of parentheses, write a function to generate all combinations of wellformed parentheses. For example, given n = 3, a solution set is "((()))", "(()())", "(())()", "()(())", "()()()"  generate_parenthesis.cpp 
Given an array containing n distinct numbers taken from 0, 1, 2, ..., n, find the one that is missing from the array.For example, Given nums = [0, 1, 3] return 2.  missing_number.cpp 
Suppose a sorted array is rotated at some pivot unknown to you beforehand. (i.e., 0 1 2 4 5 6 7 might become 4 5 6 7 0 1 2). Find the minimum element. You may assume no duplicate exists in the array.  find_min_rotated.cpp 
Given an array S of n integers, find three integers in S such that the sum is closest to a given number, target. Return the sum of the three integers. You may assume that each input would have exactly one solution.  threeSumClosest.cpp 
Given n nonnegative integers a_{1}, a_{2}, ..., a_{n}, where each represents a point at coordinate (i, a_{i}). n vertical lines are drawn such that the two endpoints of line i is at (i, a_{i}) and (i, 0). Find two lines, which together with xaxis forms a container, such that the container contains the most water. Note: You may not slant the container.  maxArea.cpp 
Given a binary tree containing digits from 09 only, each roottoleaf path could represent a number. An example is the roottoleaf path 1>2>3 which represents the number 123. Find the total sum of all roottoleaf numbers. Example in solution comments  sumRootToLeafNumbers.cpp 
Say you have an array for which the ith element is the price of a given stock on day i. If you were only permitted to complete at most one transaction (ie, buy one and sell one share of the stock), design an algorithm to find the maximum profit.  maxProfitStock.cpp 
Given a m x n grid filled with nonnegative numbers, find a path from top left to bottom right which minimizes the sum of all numbers along its path. Note: You can only move either down or right at any point in time.  minPath.cpp 
Count the number of prime numbers less than a nonnegative number, n.  countPrimes.cpp 
Find all possible combinations of k numbers that add up to a number n, given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers. Ensure that numbers within the set are sorted in ascending order. Example : for k = 3, n = 9 result would be [[1,2,6], [1,3,5], [2,3,4]], similarly for k = 3, n = 7, result would be [[1,2,4]].  combinationSum3.cpp 
Given a nonnegative integer num, repeatedly add all its digits until the result has only one digit. For example: Given num = 38, the process is like: 3 + 8 = 11, 1 + 1 = 2. Since 2 has only one digit, return it. Follow up: Could you do it without any loop/recursion in O(1) runtime?  addDigits.cpp 
Given a matrix with cell values 0 or 1. Find the length of the shortest path from (a1, b1) to (a2, b2), such that path can only be constructed through cells which have value 1 and you can only travel in 4 possible directions, i.e. left, right, up and down.  shortest_path_maze.cpp 
The Hamming distance between two integers is the number of positions at which the corresponding bits are different. Given two integers x and y, calculate the Hamming distance.  hamming_distance.cpp 
Given two binary trees and imagine that when you put one of them to cover the other, some nodes of the two trees are overlapped while the others are not. You need to merge them into a new binary tree. The merge rule is that if two nodes overlap, then sum node values up as the new value of the merged node. Otherwise, the NOT null node will be used as the node of new tree.  merge_trees.cpp 
Write a function that takes a string as input and reverse only the vowels of a string.  reverse_vowels.cpp 
Given a string, sort it in decreasing order based on the frequency of characters.For example:

sortCharByFrequency.cpp 
Product of Array Except Self. Given an array of n integers where n > 1, nums, return an array output such that output[i] is equal to the product of all the elements of nums except nums[i].  product_except_self.cpp 
Given a sorted array, remove duplicates in place and return the new length. It doesn't matter what is in array beyond the unique elements size. Expected O(1) space and O(n) time complexity.  remove_duplicates.cpp 
Count the number of islands in a grid. Given a grid representing 1 as land body, and 0 as water body, determine the number of islands (more details in problem comments)  count_islands.cpp 
Find median from a data stream. Design a data structure that supports addNum to add a number to the stream, and findMedian to return the median of the current numbers seen so far. Also, if the count of numbers is even, return average of two middle elements, return median otherwise.  median_stream.cpp 
Remove the minimum number of invalid parentheses in order to make the input string valid. Return all possible results. Note: The input string may contain letters other than the parentheses ( and )  remove_invalid_parenthesis.cpp 
Given an array and a value, remove all instances of that value inplace and return the new length. Do not allocate extra space for another array, you must do this by modifying the input array inplace with O(1) extra memory. The order of elements can be changed. It doesn't matter what you leave beyond the new length.  remove_element.cpp 
Find intersection of two arrays/vectors, Given two vectors find the result of their interaction. The result should only contain unique characters and can be in any order  intersection_of_array.cpp 
Given a pattern and a string str, find if str follows the same pattern. Here follow means a full match, such that there is a bijection between a letter in pattern and a nonempty word in str. example:  
pattern = "abba", str = "dog cat cat dog" should return true.  
pattern = "abba", str = "dog cat cat fish" should return false.  
pattern = "aaaa", str = "dog cat cat dog" should return false.  
pattern = "abba", str = "dog dog dog dog" should return false.  word_pattern.cpp 
You are provided a vector of numbers, where each number represents  
price of a stock on ith day. If you are permitted to only complete  
one transaction per day (i.e buy one and sell one stock), design  
an algorithm to find the maximum profit.  best_time_to_buy_sell.cpp 
Given a sentence, reverse the order of characters in each word within a sentence while still preserving whitespace and initial word order.  
Example:  
Input: She loves chocolate  
Output: ehs sevol etalocohc  reverse_words.cpp 