43. Multiply Strings

class Solution {
    public String multiply(String num1, String num2) {
        if(num1 == null || num2 == null){
            return null;
        }
        if(num1.charAt(0) == '0' || num2.charAt(0) == '0'){
            return "0";
        }
        int[] retval = new int[num1.length() + num2.length()];
        StringBuilder builder = new StringBuilder();
        int extraValue = 0;
        
        for(int i = num1.length() - 1; i >= 0; i--){
            for(int j = num2.length() - 1; j >= 0; j--){
                //Find total of two numbers
                int total = (Character.getNumericValue(num1.charAt(i))) * (Character.getNumericValue(num2.charAt(j))) + extraValue;
                //Figure out where to insert and insert it
                int spotToInsertValue = retval.length - ((num1.length() - i) + (num2.length() - j)) + 1;
                retval[spotToInsertValue] += total;
            }             
        }
        for(int i = retval.length - 1; i >= 0; i--){
            retval[i] += extraValue;
            extraValue = retval[i] / 10;
            retval[i] = retval[i] % 10;
        }
        for(int i = 0; i < retval.length; i++){
            if(retval[i] != 0){
                for(int j = i; j < retval.length; j++){
                    builder.append((char)(retval[j] + 48));
                }
                break;
            }
        }
        return builder.toString();   
    }
}