7. Two Pointers
Opposite ends, same direction, and merging — two indexes instead of nested loops.
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Understand the topic
Two pointers means using two indexes instead of a nested loop. It shows up in three shapes: pointers at both ends moving inward (sorted pairs, palindromes), a slow "write" pointer and a fast "read" pointer moving the same way (removing items in place), and one pointer per array (merging).
The key question every time: after looking at the two pointed values, which pointer can I safely move, and why can't I miss the answer by moving it?
| # | Problem | Pattern | Level | ||
|---|---|---|---|---|---|
| 1 | Remove Duplicates from Sorted Array #26 | Read/write pointers | Easy | LeetCode ↗ | |
| 2 | Remove Element #27 | Read/write pointers | Easy | LeetCode ↗ | |
| 3 | Move Zeroes #283 | Read/write pointers | Easy | LeetCode ↗ | |
| 4 | Valid Palindrome #125 | Opposite ends | Easy | LeetCode ↗ | |
| 5 | Squares of a Sorted Array #977 | Opposite ends, fill from back | Easy | LeetCode ↗ | |
| 6 | Is Subsequence #392 | Same direction | Easy | LeetCode ↗ | |
| 7 | Two Sum II - Input Array Is Sorted #167 | Opposite ends | Medium | LeetCode ↗ | |
| 8 | 3Sum #15 | Sort + two pointers | Medium | LeetCode ↗ | |
| 9 | 4Sum #18 | Sort + two pointers | Medium | LeetCode ↗ | |
| 10 | Container With Most Water #11 | Opposite ends, move the shorter | Medium | LeetCode ↗ | |
| 11 | Remove Duplicates from Sorted Array II #80 | Read/write pointers | Medium | LeetCode ↗ | |
| 12 | Boats to Save People #881 | Sort + opposite ends | Medium | LeetCode ↗ | |
| 13 | Trapping Rain Water #42 | Opposite ends with max heights | Hard | LeetCode ↗ |
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