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LeetCode 167, Medium. Topics: Array, Two Pointers, Binary Search. View on LeetCode. Generate this problem as a practice environment: tested reference solution, 14 parametrized pytest cases, and a playground notebook:

Problem

Given a 1-indexed array of integers numbers that is already sorted in non-decreasing order, find two numbers such that they add up to a specific target number. Let these two numbers be numbers[index1] and numbers[index2] where 1 <= index1 < index2 <= numbers.length. Return the indices of the two numbers index1 and index2, each incremented by one, as an integer array [index1, index2] of length 2. The tests are generated such that there is exactly one solution. You may not use the same element twice. Your solution must use only constant extra space.

Examples

Explanation: The sum of 2 and 7 is 9. Therefore index1 = 1, index2 = 2. We return [1, 2].
Explanation: The sum of 2 and 4 is 6. Therefore index1 = 1, index2 = 3. We return [1, 3].
Explanation: The sum of -1 and 0 is -1. Therefore index1 = 1, index2 = 2. We return [1, 2].

Constraints

  • 2 <= numbers.length <= 3 * 10^4
  • -1000 <= numbers[i] <= 1000
  • numbers is sorted in non-decreasing order.
  • -1000 <= target <= 1000
  • The tests are generated such that there is exactly one solution.

Solution

Reference implementation from solution.py on GitHub, full suite in test_solution.py:

Complexity

Tags

NeetCode 150, NeetCode 250, NeetCode All.
Last modified on August 25, 2026