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

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 = 10899810 - 0189 = 96219621 - 1269 = 83528532 - 2358 = 6174

Sample Input 2:

2222

Sample Output 2:

2222 - 2222 = 0000

#include <iostream>#include <vector>#include <algorithm>#include <string>#include <set>using namespace std;bool cmp(char a, char b) {return a > b;}int main() {string s;cin >> s;s.insert(0, 4 - s.length(), '0');do {string a = s, b = s;sort(a.begin(), a.end(),cmp);sort(b.begin(), b.end());int result = stoi(a) - stoi(b);s = to_string(result);s.insert(0, 4 - s.length(), '0');cout << a << " - " << b << " = " << s<<endl;} while (s != "6174"&&s != "0000");return 0;}