1.js

/** ------------------------------------------------------------------------------
 * 
 * 1. Two Sum
 * Topics: Hash Table
 * https://leetcode.com/problems/two-sum/description/
 * 
 ------------------------------------------------------------------------------ */
/**
 * @param {number[]} nums
 * @param {number} target
 * @return {number[]}
 */
var twoSum = function (nums, target) {
  let hash = {}

  for (let i = 0; i < nums.length; i++) {
    const n = nums[i]
    const diff = target - n

    if (diff in hash) {
      // if (hash[diff] !== undefined) {
      return [hash[target - n], i]
    }
    hash[n] = i
  }

  return []
}

console.log(twoSum([2, 7, 11, 15], 9))

/** ------------------------------------------------------------------------------
 * 
 * use Map
 * 
 ------------------------------------------------------------------------------ */
/**
 * @param {number[]} nums
 * @param {number} target
 * @return {number[]}
 */
var twoSumMap = function (nums, target) {
  const map = new Map()

  for (let i = 0; i < nums.length; i++) {
    let complement = target - nums[i]
    if (map.has(complement)) return [map.get(complement), i]

    map.set(nums[i], i)
  }
}

console.log(twoSumMap([2, 7, 11, 15], 9))