0001. Two Sum.js

// Given an array of integers, return indices of the two numbers such that they add up to a specific target.
//
// You may assume that each input would have exactly one solution, and you may not use the same element twice.
//
// Example:
//
// Given nums = [2, 7, 11, 15], target = 9,
//
// Because nums[0] + nums[1] = 2 + 7 = 9,
// return [0, 1].

/**
 * @param {number[]} nums
 * @param {number} target
 * @return {number[]}
 */

// 1) Brute force
// Time O(n^2)
// Space O(1)
const twoSum1 = (nums, target) => {
  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];
    }
  }
};

// 2)
// Time O(n)
// Space O(n)
const twoSum = (nums, target) => {
  const map = {};
  for (let i = 0; i < nums.length; i++) {
    const diff = target - nums[i];
    if (map[diff] != null) return [map[diff], i];
    map[nums[i]] = i;
  }
};