归并排序(Merging Sort)----(排序算法十三)

1.算法原理 

2.代码实现

#include <stdio.h>

//printArray打印出数组
void printArray(int a[],int size){  
    printf("数组为：%d ",a[0]);  
    for (int i=1;i<size;i++)  
    {  
        printf(" %d ",a[i]);  
    }  
    printf("\n");  
}


//将有二个有序数列a[first...mid]和a[mid...last]合并。
void mergearray(int a[], int first, int mid, int last, int temp[])
{
	printArray(a,last);
	int i = first, j = mid + 1;
	int m = mid,  n = last;
	int k = 0;
	while (i <= m && j <= n)
	{
		if (a[i] <= a[j])
			temp[k++] = a[i++];
		else
			temp[k++] = a[j++];
	}
	while (i <= m)
		temp[k++] = a[i++];
	while (j <= n)
		temp[k++] = a[j++];
	for (i = 0; i < k; i++)
		a[first + i] = temp[i];
}

//递归归并排序
void mergesort(int a[], int first, int last, int temp[])
{
	if (first < last)
	{
		int mid = (first + last) / 2;
		mergesort(a, first, mid, temp);    //左边有序
		mergesort(a, mid + 1, last, temp); //右边有序
		mergearray(a, first, mid, last, temp); //再将二个有序数列合并
	}
}

bool MergeSort(int a[], int n)
{
	int *p = new int[n];
	if (p == NULL)
		return false;
	mergesort(a, 0, n - 1, p);
	delete[] p;
	return true;
}

void main()
{
	int  a[10] ={9,8,7,6,5,4,3,2,1,0};  
	MergeSort(a,10);
	printArray(a,10);
}

3.结果

数组为：9
数组为：8  9
数组为：7  8  9  6
数组为：7  8  9  5
数组为：5  6  7  8  9  4
数组为：5  6  7  8  9  3  4
数组为：5  6  7  8  9  2  3  4  1
数组为：5  6  7  8  9  2  3  4  0
数组为：5  6  7  8  9  0  1  2  3
数组为：0  1  2  3  4  5  6  7  8  9