[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) {
        if(a==null)
            return b;
        if(b==null)
            return a;
        int end_a=a.length()-1;
        int end_b=b.length()-1;
        int carry=0;
        String result="";
        while(end_a>=0&&end_b>=0){
            int temp=(a.charAt(end_a)-'0')+(b.charAt(end_b)-'0')+carry;
            carry=temp/2;
            temp%=2;
            result=(temp+"")+result;
            end_a--;
            end_b--;
        }
        while(end_a>=0){
            int temp=(a.charAt(end_a)-'0')+carry;
            carry=temp/2;
            temp%=2;
            result=(temp+"")+result;
            end_a--;
        }
        while(end_b>=0){
            int temp=(b.charAt(end_b)-'0')+carry;
            carry=temp/2;
            temp%=2;
            result=(temp+"")+result;
            end_b--;
        }
        if(carry!=0){
            result=(carry+"")+result;
        }
        return result;
    }
}