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1. Two Sum

easyAsked at Revolut

Find two indices whose values sum to a target, a Revolut warm-up that mirrors pairing two transaction legs to net a customer balance to zero.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given an array of integers nums and an integer target, return indices of two numbers that add up to target. You may assume exactly one solution exists and you may not use the same element twice.

Constraints

  • 2 <= nums.length <= 10^4
  • -10^9 <= nums[i] <= 10^9
  • Only one valid answer exists

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.

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 complement of each value as we scan. One pass, O(n).

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:

Revolut-specific tips

Revolut frames this as a debit/credit pairing exercise — explicitly call out that minor units (pence/cents) must be integers, not floats, before you write the hash map.

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Output

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