恶心人的点：输入不是四位数，计算结果不是四位数（包括递增序列数不是四位和差值不是四位）
%0xd、sprintf和sscanf是解这道题的利器

#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
bool cmp(int a, int b) {
	return a > b;
}
void newnum(char* num) {
	char result[5];
	int high, low, def;
	sort(num, num + 4);
	sscanf(num, "%d", &low);
	sort(num, num + 4, cmp);
	sscanf(num, "%d", &high);
	def = high - low;
	sprintf(result, "%04d", def);
	printf("%04d - %04d = %04d", high, low, def);
	if (def != 6174) {
		printf("\n");
		newnum(result);
	}
}
int main() {
    char num[5];
    int i;
    scanf("%d", &i);
    sprintf(num, "%04d", i);
    if (num[0] == num[1] && num[0] == num[2] && num[3] == num[0]) {
        printf("%s - %s = 0000", num, num);
    }else {
        newnum(num);
    }
	return 0;
}