# Two Sum (4 Qs)

This question is kind of the classic problem in LeetCode. Though it seems like a pretty easy question, **through its extension**, we could learn a lot.

**Related problems (follow the order to solve each problem):**

* [0001. Two Sum](https://leetcode.com/problems/two-sum/) ***`Hash Table`***
* [0167. Two Sum II - Input array is sorted](https://leetcode.com/problems/two-sum-ii-input-array-is-sorted/)  ***`Two Pointer`***
* [0170. Two Sum III - Data structure design](https://leetcode.com/problems/two-sum-iii-data-structure-design/)  ***`Binary Search`***
* [0653. Two Sum IV - Input is a BST](https://leetcode.com/problems/two-sum-iv-input-is-a-bst/)  ***`Binary Search Tree + Stack`***