Leetcode-Add Binary

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

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

Solution:

public class Solution {
    public String addBinary(String a, String b) {
        String res = "";
        int index1 = a.length()-1;
        int index2 = b.length()-1;
        int carry = 0;
        while (index1>=0 || index2>=0){
            int val = 0;
            if (index1<0){
                val = (b.charAt(index2)=='0')? 0 : 1;
                val += carry;                
                index2--;
            } else if (index2<0){
                val = (a.charAt(index1)=='0')? 0 : 1;
                val += carry;                
                index1--;
            } else {
                val = (a.charAt(index1)=='0')? 0 : 1;
                val += (b.charAt(index2)=='0')? 0 : 1;
                val += carry;                
                index1--;
                index2--;
            }
            carry = val/2;
            val = val%2;
            res = Integer.toString(val)+res;
        }
        if (carry>0) res = Integer.toString(carry)+res;
        return res;
    }
}