Algorithmseasy

What is the two pointers technique?

Answer

You keep two indices (e.g., left/right) and move them based on a rule, often on a sorted array or a sliding window. It can reduce nested loops to O(n) in many problems.

function hasPairSum(arr: number[], target: number) {
  let l = 0;
  let r = arr.length - 1;

  while (l < r) {
    const sum = arr[l] + arr[r];
    if (sum === target) return true;
    if (sum < target) l++;
    else r--;
  }

  return false;
}

Advanced answer

Deep dive

Two pointers is a technique family: you maintain two positions and an invariant that lets you move one pointer safely.

Common variants:

Left/right on a sorted array (2-sum, pair constraints).
Sliding window (expand right, shrink left while condition fails).
Slow/fast pointer (cycle detection, middle of list).

How to use it well

Define the invariant (what the window/search space guarantees).
Ensure each step moves at least one pointer; total pointer moves stay O(n).

Common pitfalls

Sliding window breaks with negative numbers unless the invariant still holds.
Forgetting sorted preconditions for left/right patterns.
Off-by-one mistakes when updating boundaries.

Answer

Advanced answer

Deep dive

How to use it well

Common pitfalls

Related questions