Two Pointers

Two pointers is the pattern you reach for when the input has order — sorted arrays, palindromes, or "find a pair that sums to X."

When it applies

• Input is sorted (or can be sorted once).
• You need a pair/triplet with a target sum.
• You're checking symmetry (palindrome, valid parentheses).

Classic: pair with target sum

Given a sorted array, find two numbers that add up to target.

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

Complexity

In an interview, say the sorted assumption out loud. If the array isn't sorted, ask whether you may sort — that's often the difference between O(n) and O(n log n).