Add Binary

Given two binary strings, return their sum (also a binary string).

For example,
a = "11"
b = "1"
Return "100".

StringBuilder +  reverse method

public class Solution {
    public String addBinary(String a, String b) {
        if(a == null || a.equals("")) return b;
        if(b == null || b.equals("")) return a;
        
        StringBuilder a1 = new StringBuilder(a);
        a1.reverse();
        StringBuilder b1 = new StringBuilder(b);
        b1.reverse();
        
        StringBuilder result = new StringBuilder();
        int len = Math.max(a.length(), b.length());
        int carry = 0;
        for(int i = 0; i < len ; i++){
            int t1 = (i >= a.length()? 0 : (a1.charAt(i) - '0'));
            int t2 = (i >= b.length()? 0 : (b1.charAt(i) - '0'));
            int t3 = t1+t2+carry;
            carry = t3/2;
            t3 = t3%2;
            result.append(t3);
        }
        
        if(carry != 0){
            result.append(carry);
        }
        
        result.reverse();
        return result.toString();
    }
}