Map算法

求两数之和

https://leetcode.cn/problems/two-sum/

function towSum(arr, target) {
  const map = new Map();
  for(let i = 0, len = arr.length; i< len; i++) {
    const it = arr[i];
    const next = target - it;
    if(map.has(next)) {
      return [map.get(next), i]
    } else {
      map.set(next, i)
    }
  }
}