排序算法之归并排序

归并排序原理即将两个有序的数组合并成一个，归并排序有两种方法：递归和循环。

/*递归方法*/

void Merge(int TR1[], int TR2[], int low, int mid, int high)
{//将TR2归并入TR1中
	int pos1 = low;
	int pos2 = mid + 1;
	for (int i = low; i <= high; ++i)
	{
		if (pos1 <= mid && pos2 <= high)
		{		
			if (TR2[pos1]>TR2[pos2])
			{
				TR1[i] = TR2[pos2];
				pos2++;
			}
			else
			{
				TR1[i] = TR2[pos1];
				pos1++;
			}
	    }
		else if (pos1<=mid)
		{
			TR1[i] = TR2[pos1++];
		}
		else
		{
			TR1[i] = TR2[pos2++];
		}		
	}
}
void Msort(int SR[],int TR1[], int low, int high)
{
	int TR2[MAXSIZE + 1];
	if (low < high)
	{//先所有存放到TR2中，再由TR2归并到TR1
		int mid = (low + high) / 2;
		Msort(SR,TR2,low, mid);
		Msort(SR,TR2,mid + 1, high);
		Merge(TR1, TR2, low, mid, high);
	}
	else
	{
		TR1[low] = SR[low];
	}
}
void MergeSort1(SqList* list)
{
	Msort(list->data,list->data,0,list->length-1);
}

/*循环方法*/
void MergePass(int TR[], int SR[], int k, int length)
{
	int i = 0;
	while (i <= length-2*k+1)
	{//两两合并
		Merge(TR, SR, i, i + k-1, i + 2 * k - 1);
		i = i + 2 * k;
	}

	if (i <= length - k + 1)
	{//说明后面还剩两个子数组，一个是完整的k个，还有一个小于k
		Merge(TR, SR, i, i + k - 1, length);
	}
	else
	{//最后仅仅剩一个子数组，
		for (int j = i; j <= length; j++)
		{
			TR[j] = SR[j];
		}
	}
}
void MergeSort2(SqList* list)
{/*从最小的序列開始归并，直至完毕归并*/
	int* TR = new int[list->length];
	int k = 1;
	while (k < list->length)
	{//两次转存。先从data转存到TR,再从TR转存到data; 
		MergePass(TR, list->data, k, list->length - 1);
		k = k * 2;
		MergePass(list->data, TR, k, list->length - 1);
		k = k * 2;
	}
}