43. Multiply Strings字符串相乘

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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 == "" || num2 == "")

            return "";

        vector<int> sums(siz1+siz2, );

        int mul = ;

        int p1, p2;

        int sum;

        string ans = "";

        for(int i = siz1-; i>=; i--)

        {

            for(int j = siz2-; j>=; j--)

            {

                mul = (int)(num1[i]-'') * (int)(num2[j]-'');

                p1 = i+j;

                p2 = i+j+;

                sum = sums[p2] + mul;

                sums[p1] += sum / ;

                sums[p2] = sum % ;

            }

        }

        for(auto i : sums)

        {

            ans.append(to_string(i));

        }

        if(ans[] == '')

            ans = ans.substr(, ans.size()-);

        return ans;

    }

};