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1. Two Sum

easyAsked at Byju's

Given an array of integers, return indices of two numbers adding to a 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. 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], 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 one-pass

Track complements in a hash map for O(1) lookup. Single linear scan resolves each pair on encounter.

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:

Byju's-specific tips

Byju's interviewers expect you to verbalize the brute-force baseline before optimizing, since they grade strongly on clear teaching-style explanation.

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