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[LeetCode] 70. Climbing Stairs

You are climbing a staircase. It takes n steps to reach to the top. Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top? Note: Given n will be a positive in

2020-02-13

#leetcode #java #dynamic programming #math #memoization

[LeetCode] 107. Binary Tree Level Order Traversal II

Given the root of a binary tree, return the bottom-up level order traversal of its nodes’ values. (i.e., from left to right, level by level from leaf to root). Example 1:Input: root = [3,9,20,nul

2020-02-11

#leetcode #tree #bfs #java

[LeetCode] 98. Validate Binary Search Tree

Given the root of a binary tree, determine if it is a valid binary search tree (BST). A valid BST is defined as follows:The left subtree of a node contains only nodes with keys less than the node’s ke

2020-01-31

#leetcode #tree #java #javascript #inorder #bst

938. Range Sum of BST

Given the root node of a binary search tree and two integers low and high, return the sum of values of all nodes with a value in the inclusive range [low, high]. Example 1:Input: root = [10,5,15,

2020-01-30

#leetcode #tree #java #recursion #bst

[LeetCode] 169. Majority Element

Given an array nums of size n, return the majority element. The majority element is the element that appears more than ⌊n / 2⌋ times. You may assume that the majority element always exists in the

2020-01-28

#leetcode #java #javascript #array #hashmap #math

[LeetCode] 49. Group Anagrams

Given an array of strings strs, group the anagrams together. You can return the answer in any order. An Anagram is a word or phrase formed by rearranging the letters of a different word or phrase, typ

2020-01-24

#leetcode #java #javascript #array #hashmap #counting sort

[LeetCode] 451. Sort Characters By Frequency

Given a string s, sort it in decreasing order based on the frequency of characters, and return the sorted string. Example 1:Input: s = “tree”Output: “eert”Explanation: ‘e’ appears twice while ‘r’

2020-01-23

#leetcode #java #hashmap #sort #bucket sort #string #heap

[LeetCode] 138. Copy List with Random Pointer

A linked list of length n is given such that each node contains an additional random pointer, which could point to any node in the list, or null. Construct a deep copy of the list. The deep copy shoul

2020-01-18

#leetcode #java #javascript #hashmap #linked list

[LeetCode] 9. Palindrome Number

Given an integer x, return true if x is a palindrome, and false otherwise. Example 1:Input: x = 121Output: trueExplanation: 121 reads as 121 from left to right and from right to left. Example 2:I

2020-01-16

#leetcode #java #javascript #two pointer #math #palindrome

[LeetCode] 589. N-ary Tree Preorder Traversal

Given the root of an n-ary tree, return the preorder traversal of its nodes’ values. Nary-Tree input serialization is represented in their level order traversal. Each group of children is separated by

2020-01-15

#leetcode #tree #dfs #java #preorder #n-ary tree