Skip to main content

1. Two Sum

easyAsked at GitLab

Find two indices in an array whose values sum to a target.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given an integer array nums and an integer target, return indices i, j such that nums[i] + nums[j] == target. Each input has exactly one solution and you may not use the same element twice.

Constraints

  • 2 <= nums.length <= 10^4
  • -10^9 <= nums[i] <= 10^9
  • Exactly one valid pair exists

Examples

Example 1

Input
nums=[2,7,11,15], target=9
Output
[0,1]

Example 2

Input
nums=[3,2,4], target=6
Output
[1,2]

Approaches

1. Brute force

Check every pair of indices.

Time
O(n^2)
Space
O(1)
for (let i = 0; i < nums.length; i++)
  for (let j = i+1; j < nums.length; j++)
    if (nums[i]+nums[j]===target) return [i,j];

Tradeoff:

2. Hash map one-pass

Store complement -> index while scanning. Return when current value's complement is in the map.

Time
O(n)
Space
O(n)
function twoSum(nums, target) {
  const seen = new Map();
  for (let i = 0; i < nums.length; i++) {
    const c = target - nums[i];
    if (seen.has(c)) return [seen.get(c), i];
    seen.set(nums[i], i);
  }
}

Tradeoff:

GitLab-specific tips

At GitLab, narrate trade-offs out loud the way you would in a merge-request comment thread — interviewers grade async-friendly clarity as much as the optimal solution.

Solve it now

Free. No sign-up. Python and JavaScript run instantly in your browser.

Output

Press Run or Cmd+Enter to execute

Practice these live with InterviewChamp.AI

Drill Two Sum and other GitLab interview questions under real-loop conditions with instant feedback on your reasoning, complexity claims, and code.

Practice these live with InterviewChamp.AI →