歸併排序

//合併兩個有序數組

void Merge(int a[], int left, int mid, int right)
{
	assert(a);
	int* tmp = new int[right - left + 1]();
	int index = 0;

	int begin1 = left, end1 = mid;
	int begin2 = mid + 1, end2 = right;
	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++];
	}
	memcpy(a + left, tmp, sizeof(int)*(index));
	delete[]tmp;
}

歸併實現的兩種方法

（1）：遞歸（自頂向下）

void MergeSort(int a[], int left, int right)
{
	if (left < right)
	{
		int mid = left + (right - left) / 2;
		MergeSort(a, left, mid);
		MergeSort(a, mid + 1, right);
		Merge(a, left, mid, right);
	}
}

（2）：循環控制（自底向上）：

void merge_sort(int a[], int size)
{
	assert(a);
	for (int gap = 1; gap < size; gap *= 2)//gap爲子數組中元素的個數
	{
		int i = 0;
		for (; i + 2 * gap - 1 < size; i += 2 * gap)
		{
			Merge(a, i, i + gap - 1, i + 2 * gap - 1);
		}
		if (i + gap - 1 < size-1)
		{
			Merge(a, i, i + gap - 1, size - 1);
		}
	}
	
}