What is the time and space complexity of Two Sum II (Sorted)?

Time complexity: O(n). Space complexity: O(1).

Two Sum II (Sorted)

Easy

Concept

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Solution Guide: Two Sum II (Sorted)

Approach

Place one pointer at the start and one at the end of the sorted array. If the sum is too small, advance the left pointer to increase it. If too large, retreat the right pointer to decrease it. This converges on the target pair in one pass.

Complexity Analysis

Time: O(n)
Space: O(1)

Pattern

Two Pointers (Converging)

Why It Works

Sorted order guarantees that moving the left pointer right increases the sum and moving the right pointer left decreases it, so each step provably eliminates at least one candidate pair.

Updated Feb 2026