程式分析：可以用取餘符來獲取這個四位數的每一位，然後反序重新組合輸出。

#include "stdio.h"

int main(void)
{
	int num = 0, opnum = 0;
	int g = 0, s = 0, b = 0, q = 0;

	printf("Please input four numbers:");
	scanf_s("%d", &num);

	g = num % 10;
	s = num % 100 / 10;
	b = num % 1000 / 100;
	q = num % 10000 / 1000;

	opnum = g * 1000 + s * 100 + b * 10 + q;

	printf
 
("The four number on the contrary is:%d\n", opnum);

	return 0;
}