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1. Two Sum

easyAsked at Glassdoor

Find two indices in an array whose values sum to a target — Glassdoor uses this to test whether you can replace an O(n^2) scan with a hash-lookup.

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

Problem

Given an array of integers nums and an integer target, return the indices of the two numbers such that they add up to target. You may assume that 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
  • Only one valid answer 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 (i, j).

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

Store complements as you scan; one pass finds the pair.

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

Tradeoff:

Glassdoor-specific tips

Glassdoor reviewers want you to verbalize the index-by-value mapping insight before coding — this mirrors how their salary aggregation pipelines index reports by employer ID.

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Output

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