Two Sum II Input Array Is Sorted

Problem

Given a 1-indexed array of integers already sorted in non-decreasing order, find two numbers that add up to a target value and return their indices (1-indexed), using only O(1) extra space.

Examples

Constraints

• 2 <= numbers.length <= 3 * 10^4
• Exactly one solution exists.
• You may not use the same element twice.

How to approach it: the Two Pointers pattern

Two indices move through a sorted (or sortable) structure — often from opposite ends or at different speeds — to avoid the nested loops a brute-force scan would need.

Look for this pattern when

• The input is sorted, or can be sorted without losing information you need.
• You're looking for a pair, triplet, or partition that satisfies a sum or comparison condition.
• You need to compare values from both ends of a sequence (e.g. palindrome checks).

Read the full Two Pointers guide →

Video walkthroughs

• Python walkthrough on YouTube ↗
• Alternative walkthrough by Engineering Digest ↗

Original problem on LeetCode ↗

More Two Pointers problems

• Valid Palindrome Easy
• 3Sum Medium
• Container With Most Water Medium
• Trapping Rain Water Hard