LeetCode-Multiply Strings

现附上代码：

class Solution {
public:
    string multiply(string num1, string num2) {
        if (num1 == "0" || num2 == "0")return "0";
        vector<int> res (num1.size()+num2.size()-1,0);
        int len = num1.size()+num2.size()-2;
        string ans = "";
        for (int i=0;i<num1.size();++i){
            for (int j=0;j<num2.size();++j){
                res[len-i-j] += (num1[i]-'0') * (num2[j]-'0');
            }
        }
        //for_each(res.begin(),res.end(),[](int& pa){cout<<pa<<",";});
        int overflow = 0;
        int digit = 0;
        //cout<<"Here"<<endl;
        int tmp = 0;
        for (int i=0;i<=len;++i){
            tmp = res[i] + overflow;
            overflow = tmp/10;
            digit = tmp%10;
            ans.append(1,digit+'0');
        }
        while (overflow > 0){
            digit = overflow%10;
            overflow = overflow/10;
            ans.append(1,digit+'0');
        }
        reverse(ans.begin(),ans.end());
        res.clear();
        vector<int>().swap(res);
        return ans;
    }
};