网址：https://leetcode.com/problems/multiply-strings/submissions/

参考：https://leetcode.com/problems/multiply-strings/discuss/17605/Easiest-JAVA-Solution-with-Graph-Explanation

把乘法分解为最最基础的加法，从后往前循环。

class Solution {public:    string multiply(string num1, string num2) {        int siz1 = num1.size(), siz2 = num2.size();        if(num1 == "0" || num2 == "0")            return "0";        vector
     
       sums(siz1+siz2, 0);        int mul = 0;        int p1, p2;        int sum;        string ans = "";        for(int i = siz1-1; i>=0; i--)        {            for(int j = siz2-1; j>=0; j--)            {                mul = (int)(num1[i]-'0') * (int)(num2[j]-'0');                p1 = i+j;                p2 = i+j+1;                sum = sums[p2] + mul;                sums[p1] += sum / 10;                sums[p2] = sum % 10;            }        }        for(auto i : sums)        {            ans.append(to_string(i));        }        if(ans[0] == '0')            ans = ans.substr(1, ans.size()-1);        return ans;    }};