Two Pointers

When to use

Use when you are comparing values from two sides, moving inward, or scanning a sorted array with left/right indices. Classic signals: palindrome check, pair sum, removing duplicates.

Core intuition

Instead of checking every pair with nested loops O(n²), move two pointers intelligently based on the current state. Each step eliminates a chunk of work.

Template

def two_pointers(arr):
    left, right = 0, len(arr) - 1
    while left < right:
        current = arr[left] + arr[right]
        if current == target:
            return [left, right]
        elif current < target:
            left += 1
        else:
            right -= 1
    return []

Suggested problems