Add Binary (LeetCode Math)

Question: Given two binary strings, return their sum (also a binary string).
For example,
a = "11"
b = "1"
Return "100".

Idea: Brute force. Keep adding from the ends of a and b until both a  and b becomes empty and the carry is 0.

Time: O(n) Space: O(1)

Code:

 public class Solution {  
   public String addBinary(String a, String b) {  
     StringBuilder builder=new StringBuilder();  
     int i=a.length()-1;  
     int j=b.length()-1;  
     int carry=0;  
     while(i>=0||j>=0||carry>0)  
     {  
       int valA=(i<0)?0:(int)(a.charAt(i)-'0');  
       int valB=(j<0)?0:(int)(b.charAt(j)-'0');  
       int sum=valA+valB+carry;  
       carry=sum/2;  
       sum=sum%2;  
       builder.insert(0,sum);  
       i--;  
       j--;  
     }  
     return builder.toString();  
   }  
 }