C用语言排序算法-归并排序（实例）

归并排序：归并算法应用到分治策略，简单说就是把一个答问题分解成易于解决的小问题后一个个解决，最后在把小问题的一步步合并成总问题的解。这里的排序应用递归来把数组分解成一个个小数组，直到小数组的数位有序，在把有序的小数组两两合并而成有序的大数组。

#include<stdio.h>
#include <stdio.h>
#include <stdlib.h>
#define N 7
void merge(int arr[], int low, int mid, int high)
{
    
    
	int i, k;
	int *tmp = (int *)malloc((high - low + 1) * sizeof(int));
	//申请空间，使其大小为两个
	int left_low = low;
	int left_high = mid;
	int right_low = mid + 1;
	int right_high = high;

	for (k = 0; left_low <= left_high && right_low <= right_high; k++)
	{
    
      // 比较两个指针所指向的元素
		if (arr[left_low] <= arr[right_low])
		{
    
    
			tmp[k] = arr[left_low++];
		}
		else 
		{
    
    
			tmp[k] = arr[right_low++];
		}
	}

	if (left_low <= left_high) 
	{
    
      //若第一个序列有剩余，直接复制出来粘到合并序列尾
								  //memcpy(tmp+k, arr+left_low, (left_high-left_low+l)*sizeof(int));
		for (i = left_low; i <= left_high; i++)
			tmp[k++] = arr[i];
	}

	if (right_low <= right_high) 
	{
    
    
		//若第二个序列有剩余，直接复制出来粘到合并序列尾
		//memcpy(tmp+k, arr+right_low, (right_high-right_low+1)*sizeof(int));
		for (i = right_low; i <= right_high; i++)
			tmp[k++] = arr[i];
	}

	for (i = 0; i < high - low + 1; i++)
	{
    
    
		arr[low + i] = tmp[i];
	}
	free(tmp);
	return;
}

void merge_sort(int arr[], unsigned int first, unsigned int last) {
    
    
	int mid = 0;
	if (first<last) {
    
    
		mid = (first + last) / 2; /* 注意防止溢出 */
								  /*mid = first/2 + last/2;*/
								  //mid = (first & last) + ((first ^ last) >> 1);
		merge_sort(arr, first, mid);
		merge_sort(arr, mid + 1, last);
		merge(arr, first, mid, last);
	}
	return;
}

int main() {
    
    
	int i;
	int a[N] = {
    
     32,12,56,78,76,45,36 };

	printf("排序前 \n");
	for (i = 0; i<N; i++)
		printf("%d\t", a[i]);

	merge_sort(a, 0, N - 1);  // 排序

	printf("\n 排序后 \n");
	for (i = 0; i<N; i++)
		printf("%d\t", a[i]); printf("\n");

	system("pause");
	return 0;
}