题解 | #大数乘法#

class Solution {
public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     * 
     * @param s string字符串 第一个整数
     * @param t string字符串 第二个整数
     * @return string字符串
     */
    string multi(string s, char b) {
        string res = "";
        if(b == '0')
            return "0";
        char flag = '0';
        while(s.size()) {
            auto a = s.back();
            s.pop_back();
            int num = (a - '0') * (b - '0') + flag - '0';
            flag = num / 10 + '0';
            num = num % 10;
            res = to_string(num) + res;
        }
        if(flag != '0')
            res = to_string(flag - '0') + res;
        return res;
    }
    string plus(string s, string t) {
        string res = "";
        char flag = '0';
        while(s.size() && t.size()) {
            auto a = s.back();
            s.pop_back();
            auto b = t.back();
            t.pop_back();
            int num = a - '0' + b - '0' + flag - '0';
            flag = num / 10 + '0';
            num = num % 10;
            res = to_string(num) + res;
        }
        while(s.size()) {
            auto a = s.back();
            s.pop_back();
            int num = a - '0' + flag - '0';
            flag = num / 10 + '0';
            num = num % 10;
            res = to_string(num) + res;
        }
        while(t.size()) {
            auto a = t.back();
            t.pop_back();
            int num = a - '0' + flag - '0';
            flag = num / 10 + '0';
            num = num % 10;
            res = to_string(num) + res;
        }
        if(flag != '0')
            res = to_string(flag - '0') + res;
        return res;
    }
    string solve(string s, string t) {
        // write code here
        int tn = t.size();
        if(s == "0" || t == "0")
            return "0";
        string res = "";
        for(int i = tn - 1; i >= 0; i--) {
            string p = multi(s, t[i]);
//             cout << "111+" << p << " ";
            for(int j = tn - 1; j > i; j--)
                p.push_back('0');
//             cout << "p + " << p << " ";
            res = plus(res, p);
//             cout << "res + " << res << " ";
        }
        return res;
    }
};