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1. Two Sum

easyAsked at Monzo

Given an array of transaction amounts and a target, find two that sum to the target.

By Alex Chen, 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 would have exactly one solution, and you may not 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

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 complement

Store seen numbers in a map; for each new number check if target - num exists in the map.

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:

Monzo-specific tips

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