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1. Two Sum

easyAsked at Spotify

Given an array of integers and a target, return indices of two numbers that sum to the target.

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. Each input has exactly one solution and you cannot use the same element twice.

Constraints

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

Store seen numbers and look up complement in O(1). Single pass.

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);
  }
  return [];
}

Tradeoff:

Spotify-specific tips

Spotify backend interviews use this warm-up to gauge fluency before diving into recommendation or playlist-graph problems, so verbalize the hash-map insight quickly and move on.

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