mergesort分治排序 - 先产生1000个随机数，连续分为3组，直到每组只剩k个元素，分别排列每组数，再用分治法合并数组-mergesort- randomly generate 1000 numbers, then split it into thirds, subarray is of size- k, stop the recursion and use insertion-sort to sort the subarray, then merge sort