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Problem Statement:     Given a tree, task is to find out the path with maximum sum of the elements along the path.  The path can be anywhere in the tree and need not go along the root and the nodes shouldn't be skipped in the path. Example 1: Input: [1,2,3] 1 / \ 2 3 Output: 6 Example 2: Input: [-10,9,20,null,null,15,7]   -10    / \   9   20     /  \     15   7 Output: 42  Approach to the solution:   Approach here is simple, first traverse the tree in post order manner. Keep track of maximum values of left and right. Keep track of maximum values of left side and right side. Compare and keep track of maximum values between maximum side max and sub tree. Keep track of global maximum sum of the path in one variable Compare global maximum sum with the sum which got from the above process. Return that maximum sum. This maximum sum is the maximum path of the sum ...

  Problem Statement: Given a binary search tree. Program should return Kth Smallest element in the binary search tree. Suppose if the binary tree is represented as: 3,1,4,null, 2 Then 1st smallest element is 1, second smallest element is 2 and 3rd smallest element is 3.   Solution with Explanation: The binary tree consists of elements in sorted order. The easiest way of finding solution is insert the elements in the priority queue and then  pop out k-1 elements, then top element of the priority queue is the kth smallest element of the tree. Optimized solution is doing in-order traversal and keeping track of the count of number of elements visited.  After visiting the left most element of the binary tree, increase the count to 1 and traverse back in in-order manner and increase the count. Whenever the count is reached to k return that element. Complexity Explanation: As the traversal is in-order, each node is visited only once. The traversal continues till the Kth el...

Problem Statement: Given an array which consists of daily temperatures, find out what is the warmth upcoming day in the list. If there is no such day exist return 0. Suppose the array of temperature is: 60, 70, 80, 77, 73, 79, 81, 75 Answer should be 1, 1, 4, 2, 1, 1, 0, 0. Explanation: Immediate warmth day after 60 is 70 which is next day, so 1. Same for 70 as well. But for 80 next warmth day is 81 which is after 4 days, for 77 it is 79 which is the second day, for 73 it is 79 which is next day. But there is no day in the list where temperature is greater than 81 so insert 0 same for 75 as well. Solution with Explanation: The idea is to use stack data structure to keep track of which we didn't find the warmth temperature in next day. First day temperature is pushed into the stack On second day we check the top value of stack is less than the current day temperature. If yes, then we found the immediate warmth day for the day which is on top of the stack. We pop that and check wheth...

Problem Statement: Given a tree return inorder traversal of a binary tree. Note: Solve it using both recursion and iterative approach. Approach - 1(Recursive approach): Recursive approach is very simple. Inorder traversal is traversing left sub tree first Then visiting root and finally visiting right sub tree. Break the recursion whenever there is no node available. Code in C++: class Solution {         void traverse(TreeNode* node, vector<int> &res){             if(!node)                 return;             traverse(node->left, res);                        res.push_back(node->val);           ...

LeetCode Problem #687 Given a binary tree find the longest possible path with same node values. The length of the path is determined the number of edges between the node. Example 1: Input: 5 / \ 4 5 / \ \ 1 1 5 Output:  2   Example 2: Input: 1 / \ 4 5 / \ \ 4 4 5 Output:  2 Solution in C++: /**  * Definition for a binary tree node.  * struct TreeNode {  *     int val;  *     TreeNode *left;  *     TreeNode *right;  *     TreeNode() : val(0), left(nullptr), right(nullptr) {}  *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}  *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}  * };  */ class Solution ...

Problem Statement: Validate whether given three set of combination leads to valid Tic-Tac-Toe  pattern or not. Given strings consists of 'x', 'o' or ' '. Assume that game starts from 'x'. So, first players1 puts 'x' in the game then player2 takes the turn and inserts 'o'. The game will continue until one of the players win. Our task is to find out whether given pattern is possible at any point of time of play. Return true if the given pattern is possible else return false. Example 1: Input: ['XXX', '   ', 'OOO'] Output:   false Explanation: The given pattern is not possible because once the player 1 put three X in a row which is in the first row game is over, player 2 doesn't get a chance to put thrid O. So, this pattern is not possible. Example 2: Input: ['XOX', 'OOX ', 'XO '] Output:   true Explanation: The given pattern is possible. Example 3: Input: ['OXX', 'XOX', ...

Problem Statement: Recover binary tree from its given preorder traversal string. The string is given in the format: Dashes followed by value. The number of dashes convey its depth and value refers to the node value. Example 1: Input: string = "1-2--3--4-5--6--7"; Output: [1,2,5,3,4,6,7] Example 2: Input: string = "1-2--3---4-5--6---7"; Output: [1,2,5,3,null, 6, null, 4, null, 7] Example 3: Input: string = "1-401--349---90--88"; Output: [1,401,null,349,88,90]. Approach to the solution: Calculate value and its depth Check if right child is present. If yes then move to right child then go to step 2 again until depth of the node is reached. Else move to left and then go to step 2 again until depth of the node is reached. Once the calculated depth is reached. If left child of the node is null, create a new node and assign left child of the node. Else create a new node and assign it to right child. Continue from step 1 again until end of the string is reached. S...

Problem Statement: A chef has collected the data on the review for his dishes. Our Chef will take just 1 unit of time to prepare a dish. Our job is to tell him the dishes he has to make in the order to achieve maximum benefit. The maximum benefit is calculated using the formula time[i] * (review ratings). Example 1: Input: reviews = [-1, -10, -9, 0, 5] Output: 14 Explanation: Considering the dishes in the order of -1, 0 ,5 the calculation will be (-1 * 1 + 0 * 2 + 5 * 3) = 14 Example 2: Input: reviews = [6,5,4] Output: 32 Explanation: Considering the dishes in the order of 4, 5, 6 the calculation will be (4 * 1 + 5 * 2 + 6 * 3) = 32 Approach to the solution: Sort the given reviews so that we can concentrate only on maximum benefited reviews. Make cumulative sums from the end. This will help in deciding till which we have to consider the summation. Now start from the end at add the previous array of cumulative sums until a negative number is encountered. We have to iterate in reverse or...

Problem Statement: Design basic twitter which lets user follow and unfollow other users and show the latest new feed related to the current user and the user followers. Implement the following APIs void postTweet(int userId, int tweetId):     Stores the tweetId against the user ID. void follow(int followerId, int followeeId):     Marks that follower ID as following followee ID void unfollow(int followerId, int followeeId):     Marks that follower ID as unfollowing followee ID vector<int> getNewsFeed(int userId):     Returns the set of the latest 10 tweetIDs which include the current user tweetIDs and tweetIDs of the user that the follower if following. Approach to the problem: First we need to store the user IDs of the people a particular user is following To store that we can use map. To optimize things instead of storing list of followers, it is better to store them in a set for quicker access. So the followers data str...

Problem Statement: Given an matrix of size m * n which is sorted in rows wise in increasing order. Return the Kth smallest sum of the elements by choosing by atmost 1 element from each row. Example 1: Input: mat = [               [1, 2, 3],               [4, 5, 6],               [7, 8, 9]                ] k = 2; Output: 13 (sum of(2,4,7)) Example 2: Input: mat = [               [1, 2, 3],               [4, 5, 6],               [7, 8, 9]                ] k = 3; Output: 14(sum of (3, 4, 7)) Approach to the solution: Take the first row and initialize to an 1D vector From second row on wards add all possible sums of first row and second ...

Problem Statement: Give an array of integers return the k most frequent elements. Example 1: Input: [10, 10, 10, 20, 20, 20, 20, 30, 30] k = 2 Output: [20, 10] Example 2: Input: [10, 10, 30] k = 1 Output: [10] Solution in C++:

Given n non-negative integers representing the histogram's bar height where the width of each bar is 1, find the area of largest rectangle in the histogram.   Above is a histogram where width of each bar is 1, given height = [2,1,5,6,2,3] .   The largest rectangle is shown in the shaded area, which has area = 10 unit.   Example: Input: [2,1,5,6,2,3] Output: 10 Solution in C++:

Given a non-empty binary tree, find the maximum path sum. For this problem, a path is defined as any sequence of nodes from some starting node to any node in the tree along the parent-child connections. The path must contain at least one node and does not need to go through the root. Example 1: Input: [1,2,3] 1 / \ 2 3 Output: 6 Example 2: Input: [-10,9,20,null,null,15,7]   -10    / \   9   20     /  \     15   7 Output: 42 Solution in C++:

Given an integer array of digits , return the largest multiple of three that can be formed by concatenating some of the given digits in any order. Since the answer may not fit in an integer data type, return the answer as a string. If there is no answer return an empty string.   Example 1: Input: digits = [8,1,9] Output: "981" Example 2: Input: digits = [8,6,7,1,0] Output: "8760" Example 3: Input: digits = [1] Output: "" Example 4: Input: digits = [0,0,0,0,0,0] Output: "0"   Constraints: 1 <= digits.length <= 10^4 0 <= digits[i] <= 9 The returning answer must not contain unnecessary leading zeros. Solution in C++:

Given a blacklist  B containing unique integers from [0, N) , write a function to return a uniform random integer from [0, N) which is NOT  in B . Optimize it such that it minimizes the call to system’s Math.random() . Note: 1 <= N <= 1000000000 0 <= B.length < min(100000, N) [0, N)  does NOT include N. See interval notation . Example 1: Input: ["Solution","pick","pick","pick"] [[1,[]],[],[],[]] Output: [null,0,0,0] Example 2: Input: ["Solution","pick","pick","pick"] [[2,[]],[],[],[]] Output: [null,1,1,1] Example 3: Input: ["Solution","pick","pick","pick"] [[3,[1]],[],[],[]] Output: [null,0,0,2] Example 4: Input: ["Solution","pick","pick","pick"] [[4,[2]],[],[],[]] Output: [null,1,3,1] Explanation of Input Syntax: The input is two lists: the subroutines called and their argume...

Given an unsorted integer array, find the smallest missing positive integer. Input: [1,2,4,5] Output: 3 Input: [0,-1,-2,1,5,2]; Output: 3 Input: [0,-1,-2]; Output: 1   Approach to solve the problem: First iterate over the array and identify all the negative elements including zero. Where ever you find zero or negative element replace its value with size of array * 2. Now iterate over the array one more time and mark the value at index. As current iterator as negative of it, If its iterator value is less than size of the  array. Ex: if the array if [1, 4, 6, -1, -3], size of the array is 5. After first iteration it will be [1, 4, 6, 10, 10] (after marking the negative and zero values with double the size of the array). Next follow step 3, arr[0] = 1 (subract -1 as array index starts from zero)which is less than size of array so, => arr[arr[0]] = - arr[arr[0]]. Next arr[2] = 4 which is less than size of array, so index will be 4 - 1 = 3, so arr[3] = - arr[3] Next va...

Given a binary tree, flatten it to a linked list in-place. For example, given the following tree: 1 / \ 2 5 / \ \ 3 4 6 The flattened tree should look like: 1 \ 2 \ 3 \ 4 \ 5 \ 6 Approach to the solution: Traverse the left sub tree until the left most leaf found recursively. Store the right child in temp variable. Take that left node and make it as right child of leaf parent. Now traverse through the right child of the parent until the leaf node is found. Assign its right child as temp. Do the same thing for right sub tree as well. Solution in C++:   /** * Definition for a binary tree node. * struct TreeNode { * int val; * TreeNode *left; * TreeNode *right; * TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */ class Solution { public: void flatten(TreeNode* root) { cal(root); } void cal(TreeNode* root){ if(!root) re...

Given two strings representing two complex numbers . You need to return a string representing their multiplication. Note i 2 = -1 according to the definition. Example 1: Input: "1+1i", "1+1i" Output: "0+2i" Explanation: (1 + i) * (1 + i) = 1 + i 2 + 2 * i = 2i,  and you need convert it to the form of 0+2i.   Solution in C++:     string complexNumberMultiply(string a, string b) {         int a1 = 0;         int a2 = 0;         int b1 = 0;         int b2 = 0;         string ret;         cal(a, a1, a2);         cal(b, b1, b2);         int real = a1 * b1 - a2 * b2;         int imag = a1 * b2 + a2 * b1;      ...

A character in UTF8 can be from 1 to 4 bytes long, subjected to the following rules: For 1-byte character, the first bit is a 0, followed by its unicode code. For n-bytes character, the first n-bits are all one's, the n+1 bit is 0, followed by n-1 bytes with most significant 2 bits being 10. This is how the UTF-8 encoding would work: Char. number range | UTF-8 octet sequence (hexadecimal) | (binary) --------------------+--------------------------------------------- 0000 0000-0000 007F | 0xxxxxxx 0000 0080-0000 07FF | 110xxxxx 10xxxxxx 0000 0800-0000 FFFF | 1110xxxx 10xxxxxx 10xxxxxx 0001 0000-0010 FFFF | 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx Given an array of integers representing the data, return whether it is a valid utf-8 encoding. Note: The input is an array of integers. Only the least significant 8 bits of each integer is used to store the data. This means each integer represents only 1 byte of data. Example 1: dat...

Note: This is a companion problem to the System Design problem: Design TinyURL . TinyURL is a URL shortening service where you enter a URL such as https://leetcode.com/problems/design-tinyurl and it returns a short URL such as http://tinyurl.com/4e9iAk . Design the encode and decode methods for the TinyURL service. There is no restriction on how your encode/decode algorithm should work. You just need to ensure that a URL can be encoded to a tiny URL and the tiny URL can be decoded to the original URL. Solution in C++: class Solution { public:     map<string, string> url;         // Encodes a URL to a shortened URL.     string encode(string longUrl) {         static int count = 0;         char *a = (char*)calloc(sizeof(char), 30);         ++count;         spr...