【算法概论】分治算法：归并排序

#include <iostream>

using namespace std;

void mergeSort(int data[], int head, int tail);	
void merge(int data[], int head, int mid, int tail);

int main()
{
	int data[4] = { 7, 5, 6, 4 };

	int len = 4;

	mergeSort(data, 0, len - 1);

	for (int i = 0; i < 4; ++i)
	{
		cout << data[i] << ' ';
	}
	cout << endl;

	return 0;

}

void mergeSort(int data[], int head, int tail)
{
	if (head < tail)
	{
		int mid = (head + tail) / 2;

		mergeSort(data, head, mid);
		mergeSort(data, mid + 1, tail);

		merge(data, head, mid, tail);
	}

	return;
}

void merge(int data[], int head, int mid, int tail)
{
	//len1和len2分别表示左边序列和右边序列的长度
	int len1 = mid - head + 1;
	int len2 = tail - mid;

	int *L = new int[len1];
	int *R = new int[len2];

	//分别给数组L和R赋值
	for (int i = 0, m = head; i < len1; ++i, ++m)
	{
		L[i] = data[m];
	}
	for (int j = 0, n = mid + 1; j < len2; ++j, ++n)
	{
		R[j] = data[n];
	}

	//比较大小，从小到大排列
	int i = 0, j = 0, k = head;
	for ( ; i < len1 && j < len2; ++k)	//i和j分别指向数组L和R的下标，k为指向数组data的下标
	{
		if (L[i] > R[j])
		{
			data[k] = R[i];
			++j;
		}
		else if (L[i] < R[j])
		{
			data[k] = L[i];
			++i;
		}
		else
		{
			data[k] = L[i];
			i += 1;
			k += 1;
			data[k] = R[j];
			j += 1;
		}
	}

	//将剩下的元素存入data中
	if (i == len1)
	{
		while (j < len2)
		{
			data[k++] = R[j++];
		}
	}
	if (j == len2)
	{
		while (i < len1)
		{
			data[k++] = L[i++];
		}
	}

	delete L;	//new的空间最后记得delete掉！
	delete R;

	return;
}