PAT甲级 1069 The Black Hole of Numbers (20 分)

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 (0,10⁴).

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

#include <iostream>
#include <algorithm>

using namespace std;

int main() {
    string s;
    cin >> s;
    s.insert(0, 4 - s.size(), '0');
    do {
        sort(s.begin(), s.end(), greater<char>());
        string max_number = s;
        sort(s.begin(), s.end(), less<char>());
        string min_number = s;
        int result = stoi(max_number) - stoi(min_number);
        s = to_string(result);
        s.insert(0, 4 - s.size(), '0');
        cout << max_number << " - " << min_number << " = " << s << endl;
        if (!result)
            break;
    } while (s != "6174");

    return 0;
}