43. Multiply Strings

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

class Solution {
public:
    string res;
    string multiply(string num1, string num2) {
        res = string(num1.length() + num2.length(), '0');
        for (int i = num2.length()-1; i >= 0; i--) {
            multiply(num1, num2[i]-'0', num2.length()-1 - i);
        }
        
        reverse(res.begin(), res.end());
        int z = 0;
        while (z < res.length()-1 && res[z] == '0') z++;
        res = res.substr(z);
        return res;
    }
    void multiply(const string& num1, int m, int offset) {
        int carry = 0, i = offset;
        for (int n = num1.length()-1; n >= 0; n--, i++) {
            int cur = num1[n] - '0';
            int val = cur * m + carry + res[i]-'0';
            res[i] = val % 10 + '0';
            carry = val / 10;
        }
        if (carry)
            res[i] = carry + '0';
    }
};