Add Binary

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

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

solution:
Basic implementation question.
Starts from the end of String. Be care of empty String case.
Tips: use StringBuilder, its append is faster than String1 + String2. Also, it has .reverse().

public class Solution {
     public String addBinary(String a, String b) {
        StringBuilder str = new StringBuilder();
        int carry = 0;
        int i = a.length() - 1;
        int j = b.length() - 1;
        while(i >= 0 || j >= 0){
            int num_a = 0, num_b = 0;
            num_a = i >= 0 ? (a.charAt(i)-'0') : 0;
            num_b = j >= 0 ? (b.charAt(j)-'0') : 0;
            int tmp = carry + num_a + num_b;
            carry = tmp / 2;
            str.append(tmp % 2);
            j--;
            i--;
        }
        if(carry == 1) str.append(carry);
        str.reverse();
        return str.toString();
    }
}