PAT Basic 1019 digital black hole (20 points)

Given any one of the digits not identical four positive integer, if we first four digits ordered by nonincreasing, then a non-descending order, and then the first number minus the second number, will get a new digital. Has been repeated so doing, we will soon be parked in the "digital black hole," he said  6174, the magic number is also called Kaprekar constant.

For example, from the 6767start, will be

7766 - 6677 = 1089
9810 - 0189 = 9621
9621 - 1269 = 8352
8532 - 2358 = 6174
7641 - 1467 = 6174
... ...

Now given any four positive integers, write a program demonstrates the process of reaching the black hole.

Input formats:

Input gives a  positive integer in the interval (  N.

Output formats:

If  N is equal to 4-bit digital full, then the output line  N - N = 0000; otherwise, the output of each step will be calculated in a row, until  6174 a difference is present, see the sample output format. Note that each digital press  4 output digit format.

Sample Input 1:

6767

Output Sample 1:

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

Sample Input 2:

2222

Output Sample 2:

2222 - 2222 = 0000

#include <iostream>
#include <algorithm>
#include <sstream>
using namespace std;
void give4(string& ini){
    for(int i=0;i<ini.length()-4;i++){
        ini+="0";
    }
}
bool cmp(char a,char b){
    return a<b;//从小到大
}
int toInt(string s){
    int sum=0;
    for(int i=0;i<s.length();i++){
        sum=sum*10+(s[i]-'0');
    }
    return sum;
}
string toStr(int a){
    stringstream ss;
    string s;
    ss<<a;
    ss>>s;
    return s;
}
int main(){
    string ini;
    cin>>ini;
    give4(ini);
    int small,big;
    while(true){
        sort(ini.begin(),ini.end(),cmp);
        small=toInt(ini);
        reverse(ini.begin(),ini.end());
        big=toInt(ini);
        printf("%04d - %04d = %04d\n",big,small,(big-small));
        if((big-small)==0||(big-small)==6174) break;
        ini=toStr((big-small));
        give4(ini);
    }
    system("pause");
    return 0;
}