LC 1. Two Sum

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/**
 * @param {number[]} nums
 * @param {number} target
 * @return {number[]}
 */
var twoSum = function(nums, target) {
  const map = new Map()
  for (let i =0; i < nums.length; i++) {
    const x = target - nums[i]
    if (map.has(x)) {
      return [map.get(x), i]
    }
    map.set(nums[i], i)
  }
};