A1069 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 (0,10^4^).

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:

Sample Output:

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

Sample Input 2:

Sample Output 2:

1	2222 - 2222 = 0000

Code

#include 
#include 
#include 
using namespace std;
const int MAXN = 100010;
const int MOD = 1000000007;
int main(){
    int i, j;
    char str[MAXN];
    int left_P[MAXN] = {0};
    int right_T[MAXN] = {0};
    gets(str);
    int len = strlen(str);
    for (i=0,j=len-1;i
        if(i>0)     left_P[i] = left_P[i - 1];
        if(j1];
        if(str[i]=='P') left_P[i]++;
        if(str[j]=='T') right_T[j]++;
    }
    int count = 0;
    for (i=0;i
        if(str[i]=='A')
            count = (count+left_P[i]*right_T[i])%MOD;
    }
    printf("%d\n", count);
    return 0;
}