两数之和

var twoSum = function (nums, target) {
    const map = new Map()
    for (let i=0; i < nums.length; i+=1) {
        const n = nums[i]
        const n2 = target -n
        if (map.has(n2)) {
            return [map.get(n2), i]
        } else {
            map.set(n, i)
        }
    }
}