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: 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: 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 in 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 ≤ a[i] ≤ 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 ≤ a[i] ≤ 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?

## Maximum Depth of Binary Tree

In this article, I explain about how to find the Maximum Depth of Binary Tree which is also called as Height of Binary Tree. Given a binary tree, find its maximum depth. The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node. Note: A … Continue reading Maximum Depth of Binary Tree

## Look and Say (Count and Say) Sequence

Day 10 Find the nth term in Count and Say Sequence. The Count and Say sequence is the sequence of below integers: "1" "11" "21" "1211" "111221" "312211" "13112221" "1113213211" "31131211131221" "13211311123113112211" "11131221133112132113212221" "3113112221232112111312211312113211" "1321132132111213122112311311222113111221131221" "11131221131211131231121113112221121321132132211331222113112211" "311311222113111231131112132112311321322112111312211312111322212311322113212221" For example: If you have "1", the next line is "11".If you have "11", the next line is … Continue reading Look and Say (Count and Say) Sequence

## 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 integers A sorted in non-decreasing order, return an array of the squares of each number, 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