1. Two Sum
easyAsked at SlackGiven an array of integers, return indices of two numbers adding up 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 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] <= 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 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 in a hash map for O(1) lookup. Iterate once and check if target-nums[i] already exists.
- 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:
Slack-specific tips
Slack interviewers value clean variable names and may ask how this scales to message-search lookups across channels.
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