## LeetCode 350. Intersection of Two Arrays II

Given two arrays, write a function to compute their intersection.

## LeetCode 347. Top K Frequent Elements

Source: https://leetcode.com/problems/top-k-frequent-elements/

Given a non-empty array of integers, return the k most frequent elements.

## LeetCode 219. Contains Duplicate II

Given an array of integers and an integer k, find out whether there are two distinct indices i and j in the array such that nums[i] = nums[j]and the absolute difference between i and j is at most k.

## LeetCode 202. Happy Number

Write an algorithm to determine if a number is “happy”.

## LeetCode 49. Group Anagrams

Given an array of strings, group anagrams together.

## LeetCode Hashmap, HashSet Problems and Solutions

Sometimes we need a data structure, it can save and retrieve data rapidly. In lots of circumstances, a hashtable is our best choice. Hashtable is a complex dynamic data structure, in the best situation is saving and retrieving time complexity is O(1).

In this article, we don’t discuss the implementation of the hashtable, We only collect the problems of the hashtable.

Some problems suit for the hashtable, like problems related string, may also suit for array solution. We may choose an array to solve these problems, but we can also use hashtable either.

## LeetCode 844. Backspace String Compare (Java)

Given two strings `S` and `T`, return if they are equal when both are typed into empty text editors. `#`means a backspace character.

## LeetCode 341. Flatten Nested List Iterator (Java)

Given a nested list of integers, implement an iterator to flatten it.

Each element is either an integer, or a list — whose elements may also be integers or other lists.

## LeetCode 232. Implement Queue using Stacks (Java)

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.