LeetCode OJ--Multiply Strings **

https://oj.leetcode.com/problems/multiply-strings/

用字符串实现大数乘法，细节题，细节很多

class Solution {
public:
    string multiply(string num1, string num2) {
        if(num1.empty() || num1.size() ==  || num2.empty() || num2.size() == )
            return "";
        //
        if(num1 == "" || num2 == "")
            return "";

        //keep num2 the smaller
        if(num1.size() < num2.size())
        {
            //exchange
            string t = num1; num1 = num2; num2 = t;
        }
        string ans = "";
        ans.resize(num1.size());

        //initialize
        for(size_t i = ; i < num1.size(); i++)
            ans[i] = '';

        int pos = ;
        int times = ;
        int sum = ;

        for(int index2 = num2.size() - ; index2 >= ; index2--)
        {
            pos = ans.size() -  - times;
            times++;
            int carry = ;
            for(int index1 = num1.size() - ; index1 >=; index1--)
            {
                sum = carry + (num1[index1] - '')*(num2[index2] - '') + ans[pos] - '';
                carry = sum/;
                ans[pos] = sum% + '';
                pos--;
                // ans has been the beginning while index1 not, and ans has been the beginning and here are carry
                if(pos == - &&(carry > || index1 != ))
                {
                    string t = "";
                    t[] = carry + '';
                    ans = t + ans;
                    carry = ;
                    pos = ;
                }
            }
        }
        return ans;
    }
};