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1. Two Sum

easyAsked at Unity

Find two array indices whose values sum to a target. Unity uses this to probe hash-map fluency for runtime entity lookups.

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

Problem

Given an integer array nums and an integer target, return indices of the two numbers such that they add up to target. Each input has 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 (i,j) for nums[i]+nums[j]===target.

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 single pass

Store complements in a Map while scanning. Each value's complement is target-num; if we've seen it, return both indices.

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:

Unity-specific tips

Unity grades for entity-component thinking: phrase the Map as a per-frame lookup table you'd reuse in a game loop instead of allocating fresh.

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Output

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