大数乘法

class Solution {
public:
    string multiply(string num1, string num2) {
        string res(num1.size()+num2.size(),'0');
        
        for(int i=num1.size()-1;i>=0;i--)
        {
            int counter=0;
            for(int j=num2.size()-1;j>=0;j--)
            {
                int temp = res[i+j+1]-'0' + (num1[i]-'0')*(num2[j]-'0')+counter;
                res[i+j+1] = temp%10 + '0';
                counter = temp/10;
            }
            res[i] = counter + '0';
        }
        
        size_t found = res.find_first_not_of("0");
        if(found!=string::npos)
        {
            return res.substr(found);
        }
        return "0";
    }
};