Hi Geeks. Welcome to 100 Days of Leetcode challenge. Design a stack that supports push, pop, top, and retrieving the minimum element in constant time. push(x) -- Push element x onto stack.pop() -- Removes the element on top of the stack.top() -- Get the top element.getMin() -- Retrieve the minimum element in the stack. Example: … Continue reading Min Stack

# Category: Data Structures and Algorithm Problems

## Path Sum II

Given a binary tree and a sum, find all root-to-leaf paths where each path's sum equals the given sum. Note: A leaf is a node with no children. Example: Given the below binary tree and sum = 22, 5 / \ 4 8 / / \ 11 13 4 / \ / \ 7 2 5 1 … Continue reading Path Sum II

## Binary Tree Zigzag Level Order Traversal

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between). For example:Given binary tree [3,9,20,null,null,15,7], 3 / \ 9 20 / \ 15 7 return its zigzag level order traversal as: [ [3], [20,9], [15,7] ] Solution Level … Continue reading Binary Tree Zigzag Level Order Traversal

## Binary Tree Paths

Given a binary tree, return all root-to-leaf paths. Note: A leaf is a node with no children. Example: Input: 1 / \ 2 3 \ 5 Output: ["1->2->5", "1->3"] Explanation: All root-to-leaf paths are: 1->2->5, 1->3 DFS Straight Forward Approach /** * Definition for a binary tree node. * public class TreeNode { * public int … Continue reading Binary Tree Paths

## Binary Tree Right Side View

Given a binary tree, imagine yourself standing on the right side of it, return the values of the nodes you can see ordered from top to bottom. Example: Input: [1,2,3,null,5,null,4] Output: [1, 3, 4] Explanation: 1 <--- / \ 2 3 <--- \ \ 5 4 <--- Solution Perform Level Order traversal with simple condition /** * Definition for … Continue reading Binary Tree Right Side View

## Binary Tree Level Order Traversal

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level). For example:Given binary tree [3,9,20,null,null,15,7], 3 / \ 9 20 / \ 15 7 return its level order traversal as: [ [3], [9,20], [15,7] ] Solution - Easy to understand /** * Definition for a binary tree … Continue reading Binary Tree Level Order Traversal

## Group Anagrams

Given an array of strings, group anagrams together. Example: Input: ["eat", "tea", "tan", "ate", "nat", "bat"], Output: [ ["ate","eat","tea"], ["nat","tan"], ["bat"] ] Note: All inputs will be in lowercase.The order of your output does not matter. Leetcode 6th day challenge C# Program Solution - Easy to Understand public class Solution { public IList<IList<string>> GroupAnagrams(string[] strs) { … Continue reading Group Anagrams

## Best Time to Buy and Sell Stock II

Say you have an array for which the ith element is the price of a given stock on day i. Design an algorithm to find the maximum profit. You may complete as many transactions as you like (i.e., buy one and sell one share of the stock multiple times). Note: You may not engage in multiple transactions at the … Continue reading Best Time to Buy and Sell Stock II

## Reverse Substrings Between Each Pair of Parentheses

Introduction Hi friends! Welcome to 100 Days of Leetcode Challenge. In this article, we going to see about Reverse Substrings Between Each Pair of Parentheses. Language I took C# programming language to solve this problem. Since language is independent of the problem. You can use whatever language you wish to solve. Only problem solving approach(logic) … Continue reading Reverse Substrings Between Each Pair of Parentheses

## Find Largest Value in Each Tree Row

Hi Geeks! Welcome to 100 Days of Leetcode Challenge. In this article, you going to see about Find Largest Value in Each Tree Row. Day 30 You need to find the largest value in each row of a binary tree. Example: Input: 1 / \ 3 2 / \ \ 5 3 9 Output: [1, … Continue reading Find Largest Value in Each Tree Row

## Move Zeroes

Given an array nums, write a function to move all 0's to the end of it while maintaining the relative order of the non-zero elements. Example: Input: [0,1,0,3,12] Output: [1,3,12,0,0] Note: You must do this in-place without making a copy of the array.Minimize the total number of operations. Approach For simple solution, we can do by introducing one variable … Continue reading Move Zeroes

## Maximum Sub Array

Hi Geeks! Welcome to 100 Days of Leetcode Challenge. In this article, we going to see about Maximum Subarray. Day 29 Given an integer array nums, find the contiguous subarray (containing at least one number) which has the largest sum and return its sum. Example: Input: [-2,1,-3,4,-1,2,1,-5,4], Output: 6 Explanation: [4,-1,2,1] has the largest sum = 6. Follow … Continue reading Maximum Sub Array

## Binary Tree Level Order Traversal II

Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root). For example:Given binary tree [3,9,20,null,null,15,7], 3 / \ 9 20 / \ 15 7 return its bottom-up level order traversal as: [ [15,7], [9,20], [3] ] Solution Iterative Approach /** * Definition … Continue reading Binary Tree Level Order Traversal II

## Happy Number

Write an algorithm to determine if a number is "happy". A happy number is a number defined by the following process: Starting with any positive integer, replace the number by the sum of the squares of its digits, and repeat the process until the number equals 1 (where it will stay), or it loops endlessly … Continue reading Happy Number

## Invert Binary Tree

Invert a binary tree. Example: Input: 4 / \ 2 7 / \ / \ 1 3 6 9 Output: 4 / \ 7 2 / \ / \ 9 6 3 1 Solution /** * Definition for a binary tree node. * public class TreeNode { * public int val; * public TreeNode left; … Continue reading Invert Binary Tree

## Root To Leaf Sum Binary Tree

Given a binary tree and a sum, determine if the tree has a root-to-leaf path such that adding up all the values along the path equals the given sum. Note: A leaf is a node with no children. Example: Given the below binary tree and sum = 22, 5 / \ 4 8 / / \ 11 … Continue reading Root To Leaf Sum Binary Tree

## Binary Tree Preorder Traversal – Iterative way

Given a binary tree, return the preorder traversal of its nodes' values. Example: Input: [1,null,2,3] 1 \ 2 / 3 Output: [1,2,3] Follow up:Â Recursive solution is trivial, could you do it iteratively? Iterative Solution /** * Definition for a binary tree node. * public class TreeNode { * public int val; * public TreeNode left; * public TreeNode right; … Continue reading Binary Tree Preorder Traversal – Iterative way

## Binary Tree Inorder Traversal

Hi Geeks! In this article, we going to look at the Binary Tree in Inorder Traversal. Given a binary tree, return the inorder traversal of its nodes' values. Example: Input: [1,null,2,3] 1 \ 2 / 3 Output: [1,3,2] Follow up:Â Recursive solution is trivial, could you do it iteratively? Solution - Recursive approach /** * Definition for a … Continue reading Binary Tree Inorder Traversal

## Implement an LRU Cache

In this article, I explained about LRU Cache implementation with example. Design and implement a data structure for Least Recently Used (LRU) cache. It should support the following operations: get and put. get(key) - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1.put(key, value) - Set or insert the value … Continue reading Implement an LRU Cache

## Implement Stack using Queues

Implement the following operations of a stack using queues. push(x) -- Push element x onto stack.pop() -- Removes the element on top of the stack.top() -- Get the top element.empty() -- Return whether the stack is empty. Example: MyStack stack = new MyStack(); stack.push(1); stack.push(2); stack.top(); // returns 2 stack.pop(); // returns 2 stack.empty(); // … Continue reading Implement Stack using Queues

## Implement Queue using Stacks

Implement the following operations of a queue using stacks. push(x) -- Push element x to the back of queue.pop() -- Removes the element from in front of queue.peek() -- Get the front element.empty() -- Return whether the queue is empty. Example: MyQueue queue = new MyQueue(); queue.push(1); queue.push(2); queue.peek(); // returns 1 queue.pop(); // returns … Continue reading Implement Queue using Stacks

## First Missing Positive

Given an unsorted integer array, find the smallest missing positive integer. Example 1: Input: [1,2,0] Output: 3 Example 2: Input: [3,4,-1,1] Output: 2 Example 3: Input: [7,8,9,11,12] Output: 1 Note: Your algorithm should run inÂ O(n) time and uses constant extra space. Solution With comments public class Solution { public int firstMissingPositive(int[] nums) { int n = … Continue reading First Missing Positive

## Find All Numbers Disappeared in an Array

Given an array of integers where 1 â‰¤ a[i] â‰¤ n (n = size of array), some elements appear twice and others appear once. Find all the elements of [1, n] inclusive that do not appear in this array. Could you do it without extra space and in O(n) runtime? You may assume the returned list does not count … Continue reading Find All Numbers Disappeared in an Array

## Find All Duplicates in an Array

Given an array of integers, 1 â‰¤ a[i] â‰¤ n (n = size of array), some elements appear twice and others appear once. Find all the elements that appear twice in this array. Could you do it without extra space and in O(n) runtime? Example: Input: [4,3,2,7,8,2,3,1] Output: [2,3] Approach When find a number i, flip the number at position i-1 to negative.If … Continue reading Find All Duplicates in an Array

## How Many Numbers Are Smaller Than the Current Number

Given the array nums, for each nums[i] find out how many numbers in the array are smaller than it. That is, for each nums[i] you have to count the number of valid j's such that j != i and nums[j] < nums[i]. Return the answer in an array. Example 1: Input: nums = [8,1,2,2,3] Output: [4,0,1,1,3] Explanation: For nums[0]=8 there exist four smaller numbers than … Continue reading How Many Numbers Are Smaller Than the Current Number

## Minimum Remove to Make Valid Parentheses

Introduction In this article, we will see about Minimum Remove to Make Valid Parentheses. We have to remove the minimum number of parentheses to make a string with valid parentheses. BACKGROUND Day 15 of 100 Days of Leetcode Programming Challenge. This problem tests our knowledge in the Stack data structure. LANGUAGE I took C# programming language … Continue reading Minimum Remove to Make Valid Parentheses

## Alphabet Board Path

INTRODUCTION In this article, we will see about Alphabet Board Path. It is one of the interesting problem, find Alphabet Board Path. BACKGROUND Day 14 of 100 Days of Leetcode Programming Challenge. This is one of the top Leetcode Problem. This problem tests our logical skills. LANGUAGE I took C# programming language to solve this … Continue reading Alphabet Board Path

## How to Reorganize String?

INTRODUCTION In this article, we will see about How to Reorganize String? It is one of the interesting problem, Reorganizing string. Background Day 13 of 100 Days of Leetcode Programming Challenge. This is one of the top Leetcode Problem. This problem tests our knowledge in string manipulations and hashing. Related Problems Top K Frequent ElementsHow … Continue reading How to Reorganize String?

## How to Sort Characters By Frequency?

Introduction In this article, we will see about How to sort characters based upon frequency? Background Day 12 of 100 Days of Leetcode Programming Challenge. This is one of the top Leetcode Problem. This problem tests our knowledge in Map data structure and Sorting. Language I took C# programming language to solve this problem. Since … Continue reading How to Sort Characters By Frequency?

## Find the Missing Number in an Array

In this article, I explained about how to find the missing number in an array. Given an array containing n distinct numbers taken from 0, 1, 2, ..., n, find the one that is missing from the array. Example 1: Input: [3,0,1] Output: 2 Example 2: Input: [9,6,4,2,3,5,7,0,1] Output: 8 Note:Your algorithm should run in linear run time … Continue reading Find the Missing Number in an Array

## Squares of a Sorted Array

Day 8 Welcome to 100 Days of Leetcode Challenge. Given an array of integersÂ AÂ sorted in non-decreasing order,Â return an array of the squares of each number,Â also in sorted non-decreasing order. Example 1: Input: [-4,-1,0,3,10] Output: [0,1,9,16,100] Example 2: Input: [-7,-3,2,3,11] Output: [4,9,9,49,121] Note: 1 <= A.length <= 10000-10000 <= A[i] <= 10000A is sorted in non-decreasing order. … Continue reading Squares of a Sorted Array

## How to Reverse Bits – Bit Manipulation

Reversing bits is one of the popular problem. How to reverse bits? In this article, we going to see about how to reverse bits in an efficient way. Reversing bits tests out understanding about bits. One of the top leetcode problem in bits section. Let's see our given problem, Reverse bits of a given 32 … Continue reading How to Reverse Bits – Bit Manipulation

## Rotate Array

Hi Geeks! Welcome to 100 Days of Leetcode Challenge. DAY 6 In this article, I'm going to explain about how to rotate an array efficiently. This is nothing but just playing with Array. Here is our given problem, Given an array, rotate the array to the right by k steps, where k is non-negative. Example 1: Input: [1,2,3,4,5,6,7] and k … Continue reading Rotate Array

## Sqrt(x)

Hi Geeks! Welcome to 100 Days of Leetcode Challenge. Day 5 Today we going to see about implementation of sqrt(x). Let's jump into our given question, Implement int sqrt(int x). Compute and return the square root of x, where x is guaranteed to be a non-negative integer. Since the return type is an integer, the decimal digits are truncated and only … Continue reading Sqrt(x)

## Evaluate Reverse Polish Notation

Hi Geeks! Welcome to 100 Days of Leetcode Challenge. Day 4 Evaluate the value of an arithmetic expression in Reverse Polish Notation. Valid operators are +, -, *, /. Each operand may be an integer or another expression. Note: Division between two integers should truncate toward zero.The given RPN expression is always valid. That means the expression would always evaluate … Continue reading Evaluate Reverse Polish Notation

## Finding all Subsets of a given set

Hi Geeks!Â Welcome to 100 Days Leetcode Challenge. Day 3 In this article, I'll explain about how to find all the subsets of a given set(power set). Here is our given problem, Given a set ofÂ distinctÂ integers,Â nums, return all possible subsets. Note: The solution set must not contain duplicate subsets. Example: Input: nums = [1,2,3] Output: [ [3], … Continue reading Finding all Subsets of a given set

## Product of Array Except Self

Hi Geeks! Welcome to 100 Days Leetcode Challenge. Day 2 Here is our given problem, Given an array nums of n integers where n > 1, return an array output such that output[i] is equal to the product of all the elements of nums except nums[i]. Example: Input: [1,2,3,4] Output: [24,12,8,6] Note: Please solve it without division and in O(n). Follow up:Could you solve it with constant space complexity? (The output array does … Continue reading Product of Array Except Self

## Top K Frequent Elements

Hi Geeks! Welcome to 100 Days Leetcode Challenge. Day 1 Here is our given problem, Given a non-empty array of integers, return the k most frequent elements. Example 1: Input: nums = [1,1,1,2,2,3], k = 2 Output: [1,2] Example 2: Input: nums = [1], k = 1 Output: [1] Note: You may assumeÂ kÂ is always valid, 1 â‰¤Â kÂ â‰¤ … Continue reading Top K Frequent Elements

## 100 Days Leetcode Challenge

Hi Geeks! Welcome to 100 Days Leetcode challenge. In these days, we going to solve Leetcode's top 100 data structures and algorithm problems. Purpose of 100 Days Leetcode Challenge The main purpose of this challenge is, as a Software Developer we should have a strong command over the algorithms and data structures.Solving these problems daily, … Continue reading 100 Days Leetcode Challenge

## Trie data structure

A trie is a tree like data structure. Trie is also called as prefix tree. In this article, we going to see three main operations of trie data structure. They are: InsertDeleteSearch TrieNode class TrieNode { int terminating; TrieNode[] trieNodes = new TrieNode[26]; public TrieNode next(final char c) { return trieNodes[c - 'a']; } } … Continue reading Trie data structure