Source: https://leetcode.com/problems/single-number/

Given a **non-empty** array of integers, every element appears *twice* except for one. Find that single one.

Source: https://leetcode.com/problems/single-number/

Given a **non-empty** array of integers, every element appears *twice* except for one. Find that single one.

Given an array of strings, group anagrams together.

Read MoreSource: https://leetcode.com/problems/valid-sudoku/

Determine if a 9×9 Sudoku board is valid. Only the filled cells need to be validated **according to the following rules**:

- Each row must contain the digits
`1-9`

without repetition. - Each column must contain the digits
`1-9`

without repetition. - Each of the 9
`3x3`

sub-boxes of the grid must contain the digits`1-9`

without repetition.

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.

- 1. Two Sum (Java)
- 3. Longest Substring Without Repeating Characters
- 30. Substring with Concatenation of All Words
- 36. Valid Sudoku
- 49. Group Anagrams
- 136. Single Number
- 202. Happy Number
- 205. Isomorphic Strings
- 217. Contains Duplicate
- 219. Contains Duplicate II
- 290. Word Pattern
- 299. Bulls and Cows
- 347. Top K Frequent Elements
- 349. Intersection of Two Arrays
- 350. Intersection of Two Arrays II
- 355. Design Twitter

8/6/2019，Bridgewater’s Ray Dalio post a new video on Youtube, talk about the impact of China’s Growth on the World Economy with his college.

Read MoreI have two Mac often use. Mac OS Catalina is working fine at my Macbook pro. But these two days, my Mac pro got a lot of problems.

Read MoreSource: https://leetcode.com/problems/backspace-string-compare/

Given two strings `S`

and `T`

, return if they are equal when both are typed into empty text editors. `#`

means a backspace character.

Source: https://leetcode.com/problems/flatten-nested-list-iterator/

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.

Read MoreSource: https://leetcode.com/problems/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.

Source: https://leetcode.com/problems/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.