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1. Two Sum

easyAsked at Riot Games

Find two indices in an array whose values sum to a target — a hash-map warm-up at Riot before pivoting to matchmaking systems.

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

Problem

Given an array of integers and a target, return the indices of the two numbers that add up to the target. Exactly one solution exists and you may not use the same element twice.

Constraints

  • 2 <= nums.length <= 10^4
  • -10^9 <= nums[i], target <= 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 one-pass

Store each value's index; for every element check whether its complement exists.

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:

Riot Games-specific tips

Riot expects you to articulate the O(n) hash-map tradeoff cleanly the same way you would justify an Elo/Glicko ranking lookup table over a naive comparison sweep.

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Output

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