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1. Two Sum

easyAsked at Brex

Given an array of integers and a target, return indices of two numbers that add to the target.

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

Problem

Return indices of the two numbers in the array that add up to the target. Exactly one solution exists and you cannot use the same element twice.

Constraints

  • 2 <= nums.length <= 10^4
  • -10^9 <= nums[i] <= 10^9
  • Exactly 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

Try every pair (i, j) and check if they sum to target.

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 one-pass

Store each value's index as you scan; look up complement in O(1). Single pass through the array.

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:

Brex-specific tips

Brex interviewers often pivot Two Sum into a transaction-pair reconciliation problem, so be ready to discuss handling duplicate amounts and stable ordering for ledger auditability.

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