递归思路：
    image.png

    image.png

    1. //归并排序
    2. let mergeSort = (arr) => {
    3.   if (arr.length <= 1) {
    4.     return arr;
    5.   }
    6.   let left = arr.slice(0, Math.floor(arr.length / 2));
    7.   let right = arr.slice(Math.floor(arr.length / 2));
    8.   return merge(mergeSort(left), mergeSort(right));
    9. };
    10. let merge = (a, b) => {
    11.   if (a.length === 0) {
    12.     return b;
    13.   }
    14.   if (b.length === 0) {
    15.     return a;
    16.   }
    17.   return a[0] < b[0]
    18.     ? [a[0]].concat(merge(a.slice(1), b))
    19.     : [b[0]].concat(merge(a, b.slice(1)));
    20.   // if (a[0] < b[0]) {
    21.   //   return [a[0]].concat(merge(a.slice(1), b));
    22.   // } else {
    23.   //   return [b[0]].concat(merge(a, b.slice(1)));
    24.   // }
    25. };