1. Two Sum
easyAsked at CourseraFind two indices whose values sum to a target — Coursera uses this to gauge hash-table fluency in a learner-pairing context.
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. You may assume each input has exactly one solution.
Constraints
2 <= nums.length <= 10^4-10^9 <= nums[i] <= 10^9Only one valid answer exists
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 complement -> index. One pass, O(1) lookup per element.
- 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:
Coursera-specific tips
Coursera interviewers like when you reframe Two Sum as a learner-pairing problem (matching two learners whose study hours sum to a target session length) and reason about the hash-map invariant out loud.
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