qsort一种实现，方便简单。使用时候只用调用sort(l,r)，l表示要排序的数组最左端的值，r为右端，例如sort(0,n-1)-qsort an implementation easy and convenient. Use only when calling sort (l, r), l want to sort the array that the value of the most left, r for the far right, such as sort (0, n-1)