pat-a1069. The Black Hole of Numbers (20)

水题，以前用的string，发现sort也能用于char数组

#include<cstdio>
#include<algorithm>
#include<cstring>
#include<functional>
using namespace std;
int main(){
char a[6];
int m,n,t;
scanf("%s",a);
int len=strlen(a);
for(int i=len;i<4;++i) a[i]='0';
do{
sort(a,a+4,greater<int>());
sscanf(a,"%d",&m);
printf("%s -",a);
sort(a,a+4);
sscanf(a,"%d",&n);
printf(" %s = %04d\n",a,m-n);
t=m-n;
sprintf(a,"%04d",t);
}while(t!=6174&&t!=0);
return 0;
}

For any 4-digit integer except the ones with all the digits being the same, if we sort the digits in non-increasing order first, and then in non-decreasing order, a new number can be obtained by taking the second number from the first one. Repeat in this manner
we will soon end up at the number 6174 -- the "black hole" of 4-digit numbers. This number is named Kaprekar Constant.

For example, start from 6767, we'll get:

7766 - 6677 = 1089

9810 - 0189 = 9621

9621 - 1269 = 8352

8532 - 2358 = 6174

7641 - 1467 = 6174

... ...

Given any 4-digit number, you are supposed to illustrate the way it gets into the black hole.

Input Specification:

Each input file contains one test case which gives a positive integer N in the range (0, 10000).

Output Specification:

If all the 4 digits of N are the same, print in one line the equation "N - N = 0000". Else print each step of calculation in a line until 6174 comes out as the difference. All the numbers must be printed as 4-digit numbers.
Sample Input 1:

6767

Sample Output 1:

7766 - 6677 = 1089
9810 - 0189 = 9621
9621 - 1269 = 8352
8532 - 2358 = 6174

Sample Input 2:

2222

Sample Output 2:

2222 - 2222 = 0000