很坑的題目，測試例子要求如果B的長度小於A，則在B前邊填上0補齊，題目中並沒有說。

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
using namespace std;

#define MAX 100+10

void zero(char a[], int d)
{
    for (int i = strlen(a)-1; i >=0; i--)
        a[i+d] = a[i];
    for (int i = 0; i < d; i++)
        a[i] = '0';
    return;
}

char A[MAX], B[MAX], C[MAX];
int main()
{
    freopen("in.txt", "r", stdin);
    //freopen("out.txt", "w", stdout);
    cin >> A >> B;

    int f = 1, i = 0, j = 0;
    if (strlen(B) < strlen(A))
        zero(B, strlen(A)-strlen(B));
    for (i = strlen(A)-1, j = strlen(B)-1; i >= 0 && j >= 0; i--, j--)
    {
        if (1 == f)//奇數位
        {
            int sum = 0;
            sum = (A[i]-'0' + B[j]-'0') % 13;
            if (sum < 10)
                C[j] = (char)(sum+'0');
            else
            {
                switch(sum)
                {
                case 10:
                    C[j] = 'J';
                    break;
                case 11:
                    C[j] = 'Q';
                    break;
                case 12:
                    C[j] = 'K';
                    break;
                }
            }
            f = 0;
        }
        else if (0 == f)
        {
            int dev;
            dev = B[j] - A[i];
            if (dev < 0)
                dev += 10;
            C[j] = (char)(dev+'0');
            f = 1;
        }
    }

    while (j >= 0)
    {
        C[j] = B[j];
        j--;
    }

    cout << C;
    return 0;
}