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1. Two Sum

easyAsked at Zoom

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 the two numbers such that they add up to target. You may assume 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.

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

Tradeoff:

2. Hash map

Store complements in a map as you scan once. Each lookup is O(1).

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:

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