1069 The Black Hole of Numbers

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 (.

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

思路：

　　注意特殊情况0000 和 6174.

Code:

#include <bits/stdc++.h>

using namespace std;

int charToInt(vector<char>& v) {
    int num = 0;
    for (int i = 0; i < 4; ++i) num = num * 10 + v[i];
    return num;
}

vector<char> intTochar(int n) {
    vector<char> num(4);
    for (int i = 3; i >= 0; --i) {
        num[i] = n % 10;
        n /= 10;
    }
    return num;
}

int main() {
    int n;
    cin >> n;
    if (n == 0) printf("%04d - %04d = %04d
", 0, 0, 0);
    if (n == 6174) printf("%04d - %04d = %04d
", 7641, 1467, 6174);
    int last = -1, temp = n;
    while (last != temp) {
        vector<char> v = intTochar(temp);
        sort(v.begin(), v.end(), greater<char>());
        int max = charToInt(v);
        sort(v.begin(), v.end(), less<char>());
        int min = charToInt(v);
        last = temp;
        temp = max - min;
        if (temp == last) break;
        printf("%04d - %04d = %04d
", max, min, temp);
    }
    return 0;
}