67. Add Binary

"11"

"1"

"100"

public class Solution {
public String addBinary(String a, String b) {
if (a == null || a == "")
return b;
if (b == null || b == "")
return a;
int carry = 0;
String ret = "";
int l1 = a.length()-1;
int l2 = b.length()-1;
while (l1 >= 0 || l2 >= 0 || carry == 1) {
if (l1 >= 0) {
carry += Integer.parseInt(a.charAt(l1)+"");
l1--;
}
if (l2 >= 0) {
carry += Integer.parseInt(b.charAt(l2)+"");
l2--;
}
ret = String.valueOf(carry%2) + ret;
carry /= 2;
}
return ret;
}
}