1. Two Sum
easyAsked at ZoomFind two indices in an array whose values sum to a target.
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 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] <= 10^9Exactly 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.
- 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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