The problem

Given an array nums and a number target, return the indices of the two numbers that add up to target.

The obvious solution checks every pair:

function twoSum(nums: number[], target: number) {
  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];
    }
  }
  return null;
}