递归思路：
    image.png

    阮一峰写法
    image.png

    1. let quickSort = (arr) => {
    2.   if (arr.length <= 1) {
    3.     return arr;
    4.   }
    5.   let pivotIndex = Math.floor(arr.length / 2);
    6.   let pivot = arr.splice(pivotIndex, 1)[0];
    7.   let left = [];
    8.   let right = [];
    9.   for (let i = 0; i < arr.length; i++) {
    10.     if (arr[i] < pivot) {
    11.       left.push(arr[i]);
    12.     } else {
    13.       right.push(arr[i]);
    14.     }
    15.   }
    16.   return quickSort(left).concat([pivot], quickSort(right));
    17. };