Biegral Blog

#include<iostream>
using namespace std;

//将有序表a[first...mid]和a[mid + 1...last]归并成a[first...last]
void merge(int a[], int first, int mid, int last)
{
	int length = last - first + 1;
	int i = first;
	int j = mid + 1;
	int k = 0;
	int *p = new int[length];

	while(i <= mid && j <= last)
	{
		if(a[i] <= a[j])  //注意，此处不能丢掉等于的情况，否则会破坏掉排序的稳定性
			p[k++] = a[i++];
		else
			p[k++] = a[j++];
	}

	while(i <= mid)   // a[mid + 1...last]已经安顿在p数组中
		p[k++] = a[i++];
	while(j <= last)  // a[first...mid]已经安顿在p数组中
		p[k++] = a[j++];

	//a[first...mid]和a[mid + 1...last]已经归并到了p数组中
	for(k = 0; k < length; k++)
		a[k + first] = p[k];

	delete [] p;
	p = NULL; //最好加这个

}

void mergeSort(int a[], int first, int last)
{
	int mid = (first + last) / 2;
	if(first < last)
	{
		mergeSort(a, first, mid);
		mergeSort(a, mid + 1, last);
		
		merge(a, first, mid, last);
	}
}

int main()
{
	int a[] = {4, 5, 1, 3, 2, 0, -3 ,-20, 100, 50};
	mergeSort(a, 0, 10 - 1);
	int i;
	for(i = 0; i < 10; i++)
		cout << a[i] << " ";
	cout << endl;

    return 0;
}