换了个写法……然后WA

#include <iostream>
#include <cstdio>
#include <string.h>
#include <string>
#include <cmath>

using namespace std;

class num
{
    public:
        int sum[5000] , len; 

        num (string in = "0")
        {
            memset(sum , 0 , sizeof(sum));

            len = in.size();

            for (int i = len - 1 ; i >= 0 ; i--)
            {
                sum[i] = in[len - i - 1] - 48;
            }
        }

        void print()
        {   
            for (int i = len - 1; i >= 0 ; i--)
            {
                //printf("%d" , sum[i]);
                putchar(sum[i]+48);
            } 
        }

        void save()
        {
            for (int i = 0 ; i <= len ; i++)
            {
                sum[i+1] += sum[i]/10;
                sum[i] %= 10;
            }

            while (!sum[len-1])
            {
                len--;
            }

            if (len == 0) len++;
        }

        void get()
        {
            string tmp;
            cin >> tmp;

            memset(sum , 0 , sizeof(sum));

            len = tmp.size();

            for (int i = len - 1 ; i >= 0 ; i--)
            {
                sum[i] = tmp[len - i - 1] - 48;
            }
        }

        int &operator [] (int i)
        {
            return sum[i];
        }
};

num operator * (num a , num b)
{
    num ou;

    ou.len = a.len + b.len;

    for (int i = a.len-1 ; i >= 0 ; i--)
    {
        for (int j = b.len-1 ; j >= 0 ; j--)
        {
            ou[i+j] += a[i]*b[i];
        }
    }

    ou.save();

    return ou;
}

int main()
{
    num a , b , c;

    a.get();b.get();

    c = a * b;
    c.print();

    return 0;
}

发现好像是计算出了问题：比如10*10 = 110.但没查出问题希望大佬救救！！！