Multiply Strings

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

 public class Solution {
    public String multiply(String num1, String num2) {
        if(num1.equals("0")||num2.equals("0")) return "0";
        int len1 = num1.length();
        int len2 = num2.length();
        int carry = 0;
        int [] res = new int[len1+len2];
        for(int i=len1-1;i>=0;i--){
            carry=0; //don't forget this
            int digit1 = num1.charAt(i)-'0';
            for(int j=len2-1;j>=0;j--){
                int digit2 = num2.charAt(j)-'0';
                res[i+j+1] +=carry+digit2*digit1;
                carry = res[i+j+1]/10;
                res[i+j+1] %=10;
            }
            if(carry>0){
                res[i]=carry;
            }
        }
        StringBuilder sb = new StringBuilder();
        if(carry>0){
            res[0]=carry;
            for(int i=0;i<res.length;i++){
                sb.append(res[i]);
            }
        }
        else{
            for(int i=1;i<res.length;i++){
                sb.append(res[i]);
            }
        }
        return sb.toString();
    }
}

 if(num1.equals("0")||num2.equals("0")) return "0";