Coding

Two pointers

Two indices moving toward/with each other on sorted data.

Coding pattern

Overview

The two-pointer technique walks two indices through a sequence — usually sorted or naturally paired — to collapse an O(n²) search into O(n). It shines on pair-sum, palindrome, and partitioning problems where each comparison lets you safely discard one side.

How it works

Coding pattern

ClientServiceEdgeData

Step by step, with examples

1
Sorted array
- Best on sorted or naturally paired data.
2
Pointer L
- Start at the beginning of the range.
3
Pointer R
- Start at the end; move inward by comparison.
4
Converge
- Stop when the pointers meet or a match is found.
- Example: Pair sum, palindrome

When to reach for it

Sorted arrays
Pair / triplet sums
Removing duplicates in place

Example problem

Pair sum in a sorted array.

Approach

Start at both ends
Move left up if sum too small, right down if too large

Solution

function pairSum(nums, target) {
  let i = 0, j = nums.length - 1;
  while (i < j) {
    const s = nums[i] + nums[j];
    if (s === target) return [i, j];
    s < target ? i++ : j--;
  }
  return [-1, -1];
}

Complexity

Time O(n), Space O(1).

Common pitfalls

Forgetting the array must be sorted
Skipping duplicates incorrectly

Where this content comes from

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