function quickSort(arr){
 if(arr.length <= 1) return arr;//判断是否有效数组
 var cut = Math.floor(arr.length/2);//取中间下标
 var left = [],right = [];
 var num = arr.splice(cut,1)[0];//取基准值
 for(var i = 0;i < arr.length;i ++){
  if(arr[i] < num){
   left.push(arr[i]);//小的放左边
  }else {
   right.push(arr[i]);//大的放右边
  }
 }
 return quickSort(left).concat(num,quickSort(right));//递归
}