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1. Two Sum

easyAsked at Udemy

Find two indices that sum to a target — Udemy's warm-up hash-map check before deeper course-search or payment questions.

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 that add up to target. Assume exactly one solution exists and the same element cannot be reused.

Constraints

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

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 each value's index as you scan and look up the complement in 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:

Udemy-specific tips

Udemy interviewers often extend this into matching a learner's budget against a basket of course prices — connect the hash-map pattern to checkout cart total problems.

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