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1. Two Sum

easyAsked at MercadoLibre

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

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

Problem

Given an array of integers nums and an integer target, return indices of two numbers such that they add up to target. You may assume exactly one solution exists and you cannot use the same element twice.

Constraints

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

Walk once, storing each value's index; check if complement already seen.

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:

MercadoLibre-specific tips

MercadoLibre warms up with this to check that you reach for a hash map fast — useful since their search and payment lookup paths rely on constant-time indexes.

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