归并排序详解-优快云博客

本文链接：https://blog.youkuaiyun.com/Iiverson2000/article/details/116757180

本文详细介绍了归并排序的两种实现方式：递归算法与非递归算法，并提供了具体的代码实现。此外，还讨论了归并排序的应用场景，包括内排序与外排序。

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归并排序

在这里插入图片描述

递归算法

void _MergeSort(int * a, int left, int right, int * tmp)
{
	if (left >= right)
		return;
	int mid = (left + right) >> 1;
	_MergeSort(a, left, mid, tmp);
	_MergeSort(a, mid + 1, right, tmp);

	int begin1 = left, end1 = mid;
	int begin2 = mid + 1, end2 = right;
	int i = left;

	while (begin1 <= end1 && begin2 <= end2)
	{
		if (a[begin1] < a[begin2])
			tmp[i++] = a[begin1++];
		else
			tmp[i++] = a[begin2++];
	}
	//拷贝回去
	for (int j = left; j <= right; ++j)
	{
		a[j] = tmp[j];
	}
}

void MergeSort(int * a, int n)
{
	int * tmp = (int *)malloc(sizeof(int)*n);
	if (tmp == NULL)
	{
		printf("malloc fail\n");
		exit(-1);
	}
	_MergeSort(a, 0, n - 1, tmp);
	free(tmp);
}

非递归算法

//void Merge(int* a, int begin, int mid, int end, int * tmp)
//{
//	int begin1 = begin;
//	int end1 = mid;
//	int begin2 = end1 + 1;
//	int end2 = end;
//	int i = begin;
//	while (begin1 <= end1 && begin2 <= end2)
//	{
//		if (a[begin1] <= a[begin2])
//			tmp[i++] = a[begin1++];
//		else
//			tmp[i++] = a[begin2++];
//	}
//	//判断是否有未合并的元素
//	if (begin1 < end1)
//	{
//		memcpy(tmp + i, a + begin1, sizeof(int)*(end1 - begin1 + 1));
//	}
//	if (begin2 < end2)
//	{
//		memcpy(tmp + i, a + begin2, sizeof(int)*(end2 - begin2 + 1));
//	}
//
//	//合并后拷贝到原数组中
//	memcpy(a + begin, tmp + begin, sizeof(int)*(end - begin) + 1);
//
//}

void _Merge(int* a, int* tmp, int begin1, int begin2, int end1, int end2)
{
	int i = begin1;
	int j = begin1;
	while (begin1 <= end1 && begin2 <= end2)
	{
		if (a[begin1] < a[begin2])
			tmp[i++] = a[begin1++];
		else
			tmp[i++] = a[begin2++];

	}
	while (begin1 <= end1)
		tmp[i++] = a[begin1++];
	while (begin2 <= end2)
		tmp[i++] = a[begin2++];

	for (; j <= end2; ++j)
		a[j] = tmp[j];
}

void MergeSortNonR(int * a, int n)
{

	int * tmp = (int *)malloc(sizeof(int)*n);
	if (tmp == NULL)
	{
		printf("malloc fail\n");
		exit(-1);
	}
	int step = 1;
	while (step < n)
	{
		for (int i = 0; i < n; i += 2 * step)
		{
			/*int begin = i;
			int mid = i + step - 1;
			if (mid >= n - 1)
				continue;
			int end = i + 2 * step - 1;
			if (end >= n)
				end = n - 1;
			Merge(a, begin, mid, end, tmp);*/
			int begin1 = i, end1 = i + step - 1, begin2 = i + step, end2 = i + 2 * step - 1;
			//如果第二个小区间不存在，就不需要归并了，结束本次循环
			if (begin2 >= n)
				break;

			//如果第二个小区间存在，但是第二个小区间不够gap个，结束位置越界了，需要修正一下
			if (end2 >= n)
				end2 = n - 1;
			_Merge(a, tmp, begin1, begin2, end1, end2);
		}
		step *= 2;
	}
	free(tmp);
}