Leetcode: 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.

Understand the problem:
The problem gives two strings with numbers only, return the multiplication of the numbers as a string as well. Note that the two numbers are positive. Also note that the numbers can be arbitrarily large, so if you convert to a number then calculate, you must handle the overflow problem. In this question, it is even harder, since the two numbers could be arbitrarily large. So we must think of another way. 

Solution:
The basic idea is to mimic how we multiply two integers. 

Code (Java):

public class Solution {
    public String multiply(String num1, String num2) {
        if (num1 == null || num1.length() == 0 || num2 == null || num2.length() == 0) {
            return "";
        }
        
        if (num1.equals("0") || num2.equals("0")) {
            return "0";
        }
        
        StringBuilder sb = new StringBuilder();
        int paddle = 0;
        
        for (int i = num1.length() - 1; i >= 0; i--) {
            StringBuilder temp = new StringBuilder();
            int carry = 0;
            for (int j = num2.length() - 1; j >= 0; j--) {
                carry += toNum(num1.charAt(i)) * toNum(num2.charAt(j));
                temp.insert(0, toChar(carry % 10));
                carry /= 10;
            }
            
            if (carry != 0) {
                temp.insert(0, toChar(carry));
            }
            
            if (sb.length() == 0) {
                sb.append(temp);
            } else {
                sb = new StringBuilder(addTwoStrings(sb, temp, ++paddle));
            }
        }
        
        return sb.toString();
    }
    
    private int toNum(char a) {
        return Character.getNumericValue(a);
    }
    
    private char toChar(int a) {
        return (char) (a + '0');
    }
    
    private StringBuilder addTwoStrings(StringBuilder str1, StringBuilder str2, int paddle) {
        for (int i = 0; i < paddle; i++) {
            str2.append("0");
        }
        
        // add integers of two strings 
        int i = str1.length() - 1;
        int j = str2.length() - 1;
        
        int carry = 0;
        StringBuilder result = new StringBuilder();
        while (i >=0 || j >= 0) {
            if (i >= 0) {
                carry += toNum(str1.charAt(i));
                i--;
            }
            
            if (j >= 0) {
                carry += toNum(str2.charAt(j));
                j--;
            }
            
            result.insert(0, toChar(carry % 10));
            carry /= 10;
        }
        
        if (carry != 0) {
            result.insert(0, toChar(carry));
        }
        
        return result;
    }
}