(Notes: "♥" means you need to buy a book from Leetcode)
| # | Title | Solution | Difficulty |
|---|---|---|---|
| 4 | Longest Substring Without Repeating Characters | php | Medium |
| 3 | Longest Substring Without Repeating Characters | php | Medium |
| 2 | Add Two Numbers | php | Medium |
| 1 | Two Sum | php | Easy |