Roadmap › Stage 2: Arrays & Strings › Pattern 6 of 53
DSA pattern guide

Array — Two Pointers

Sorted array, pairs or triplets, in-place rearranging, comparing both ends.

Stage 2 · Arrays & Strings6 problems1 Easy · 4 Medium · 1 Hard

Use it when

Sorted array, pairs or triplets, in-place rearranging, comparing both ends.

Key idea

Move two indices toward each other (or one fast, one slow) so each step rules out options without nested loops. O(n²) becomes O(n).

Practice problems, in order

  1. Move ZeroesEasy
  2. Two Sum IIMedium
  3. 3SumMedium
  4. Sort ColorsMedium
  5. Container With Most WaterMedium
  6. Trapping Rain WaterHard

How to practise this pattern

  1. Read the problem and look for the signal above before writing any code.
  2. Say the brute force out loud, with its time complexity, then ask what work it repeats.
  3. Apply the key idea and write the optimised version yourself.
  4. If you're stuck for more than 10 minutes, take one small hint, not the full solution.
  5. Move to the next problem only when you can explain why this pattern fits.

Stuck on one of these?

The AlgoSolveo extension (free, launching soon) sits beside the LeetCode problem and gives the smallest hint that gets you moving, never the full answer. Solved problems tick off on your roadmap.

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