LeetCode OJ Multiply Strings

Given two numbers represented as strings, return multiplication of the numbers as a string.

Note: The numbers can be arbitrarily large and are non-negative.

class Solution {
public:
    string multiply(string num1, string num2) {
        string ans(num1.length() + num2.length() + 1, 0);
        reverse(num1.begin(), num1.end());
        reverse(num2.begin(), num2.end());
        for (int i = 0; i < num1.size(); i++) num1[i] -= '0';
        for (int i = 0; i < num2.size(); i++) num2[i] -= '0';
        for (int i = 0; i < num1.length(); i++) {
            for (int j = 0; j < num2.length(); j++) {
                ans[i + j] += num1[i] * num2[j];
                ans[i + j + 1] += ans[i + j] / 10;
                ans[i + j] %= 10;
            }
        }
        int i;
        for (i = num1.length() + num2.length(); i > 0 && ans[i] == 0; i--);
        ans = ans.substr(0, i + 1);
        reverse(ans.begin(), ans.end());
        for (int i = 0; i < ans.size(); i++) ans[i] += '0';
        return ans;
    }
};