#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<malloc.h>
void MergeSortNonR(int* a, int n)
{
	int* tmp = (int*)malloc(sizeof(int)*n);

	int gap = 1; // 每组数据个数
	while (gap < n)
	{
		for (int i = 0; i < n; i += 2 * gap)
		{
			// [i, i+gap-1] [i+gap,i+2*gap-1]
			int begin1 = i, end1 = i + gap - 1;
			int begin2 = i + gap, end2 = i + 2 * gap - 1;

			// 归并过程中右半区间可能就不存在
			if (begin2 >= n)
				break;

			// 归并过程中右半区间算多了, 修正一下
			if (end2 >= n)
			{
				end2 = n - 1;
			}

			int index = i;
			while (begin1 <= end1 && begin2 <= end2)
			{
				if (a[begin1] < a[begin2])
				{
					tmp[index++] = a[begin1++];
				}
				else
				{
					tmp[index++] = a[begin2++];
				}
			}

			while (begin1 <= end1)
			{
				tmp[index++] = a[begin1++];
			}

			while (begin2 <= end2)
			{
				tmp[index++] = a[begin2++];
			}

			// 拷贝回去
			for (int j = i; j <= end2; ++j)
			{
				a[j] = tmp[j];
			}
		}

		gap *= 2;
	}

	free(tmp);
}
int main()
{
	int a[10] = { 3, 7, 8, 1, 4, 0, 5, 6, 2, 9 };
	int n = sizeof(a) / sizeof(int);
	MergeSortNonR(a, n);
	for (int i = 0; i < n; i++)
	{
		printf("%d ",a[i]);
	}
	return 0;
}

该算法最重要的步骤就是界限的设置