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1. Two Sum

easyAsked at Quora

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

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

Problem

Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to 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 solution 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 complement -> index as you iterate. One pass.

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

Tradeoff:

Quora-specific tips

Quora opens with Two Sum to watch how cleanly you reach for a hash lookup — the same primitive powers their question-dedup and answer-ranking inverted indexes.

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