43. Multiply Strings

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 n1 = new StringBuilder(num1);
n1 = n1.reverse();
StringBuilder n2 = new StringBuilder(num2);
n2 = n2.reverse();
int[] digit = new int[num1.length()+ num2.length() - 1];
for(int i = 0; i < n1.length(); i++) {
for(int j = 0; j < n2.length(); j++) {
digit[i + j] += ((int)n1.charAt(i)- '0') * ((int)n2.charAt(j) - '0');
}
}
int carry = 0;
StringBuilder res = new StringBuilder();
for(int i = 0 ; i < digit.length; i++) {
System.out.println(digit[i]);
}
for(int i = 0; i < digit.length; i++) {
int temp = digit[i] + carry;
res.insert(0, temp % 10);
carry = temp / 10;
}
if(carry != 0) {
res.insert(0, carry);
}
int i = 0;
while(i < res.length() && res.charAt(i) == '0') {
res.deleteCharAt(i);
}
return res.toString().equals("") ? "": res.toString();
}