1. Two Sum
easyAsked at UdemyFind two indices that sum to a target — Udemy's warm-up hash-map check before deeper course-search or payment questions.
By Sam K., 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 <= 10^9Exactly one valid pair
Examples
Example 1
Input
nums = [2,7,11,15], target = 9Output
[0,1]Example 2
Input
nums = [3,2,4], target = 6Output
[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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