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1. Two Sum

easyAsked at Klarna

Given an array of integers and a target, return indices of the two numbers 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 the indices of two numbers such that they add up to target. You may assume exactly one solution exists and you cannot 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 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 one-pass

Store complement-to-index in a map and check while iterating. Single pass.

Time
O(n)
Space
O(n)
function twoSum(nums, target) {
  const map = new Map();
  for (let i = 0; i < nums.length; i++) {
    const complement = target - nums[i];
    if (map.has(complement)) return [map.get(complement), i];
    map.set(nums[i], i);
  }
  return [];
}

Tradeoff:

Klarna-specific tips

Klarna favors hash-map fluency since their BNPL installment matching and credit-risk lookups run on amount-to-account maps with strict latency budgets.

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