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1. Two Sum

easyAsked at DigitalOcean

Find two array indices whose values sum to a target — a baseline screen at DigitalOcean for hash-map fluency that feeds into droplet-billing-line reconciliation problems.

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

Problem

Given an array of integers nums and an integer target, return the indices of the two numbers that add up to target. Each input has exactly one solution and you may not reuse the same element.

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.

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 single pass

Store complement-to-index as you scan. One pass solves it.

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);
  }
  return [];
}

Tradeoff:

DigitalOcean-specific tips

DigitalOcean values clean, single-pass solutions and expects you to articulate space/time tradeoffs because their billing-aggregation pipelines run at high volume across regions.

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Output

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