Two Pointers — the shape behind dozens of interview problems.

Two Pointers in Python

def pair_sum(nums: list[int], target: int) -> tuple[int, int] | None:
    left, right = 0, len(nums) - 1
    while left < right:
        s = nums[left] + nums[right]
        if s == target:
            return (left, right)
        if s < target:
            left += 1
        else:
            right -= 1
    return None

See language-specific tabs at /languages/python/patterns and the SQL variant at /languages/sql/patterns.

Related patterns

Frequently asked questions

What is the two pointers pattern?

Two Pointers is two cursors moving independently over a sorted or monotone structure. Time O(n) after an optional O(n log n) sort; space O(1).

When should I use two pointers?

Use it when: Input is sorted or can be sorted. Answer involves a pair, triple, or partition with a monotone predicate. You want to collapse O(n²) brute force to O(n).

What are the most common two pointers bugs?

Not sorting when the algorithm requires it. Skipping duplicates incorrectly for k-sum variants. Advancing the wrong pointer when the sum equals target (infinite loop).

What problems use the two pointers pattern?

Representative scenarios: Scale Balancer — find a pair summing to target. License Plate Mirror — check palindrome under constraints. Triple Alloy — find a triplet meeting a condition.

Is two pointers on the Blind 75?

Yes — Two Pointers is one of the core patterns behind the Blind 75 list. See /blind-75 for the complete mapping.