快速排序

let quickSort = arr => {
if (arr.length <= 1) { return arr; }
let pivotIndex = Math.floor(arr.length / 2);
let pivot = arr.splice(pivotIndex, 1)[0];
let left = [];
let right = [];
for (let i = 0; i < arr.length; i++){
if (arr[i] < pivot) { left.push(arr[i])
} else { right.push(arr[i]) }
}
return quickSort(left).concat(
[pivot], quickSort(right) )
}