LeetCode Add Binary

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

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

public class Solution {
    public String addBinary(String a, String b) {
       
       int la=a.length();
       int lb=b.length();
       char[] chars=new char[Math.max(la,lb)+1];
       int flag=0;
       int lastA=la-1;
       int lastB=lb-1;
       int c=0;
       int currA,currB;
       for (int i = chars.length-1; i >=0; i--) {
                   currA=lastA>=0?a.charAt(lastA):'0';
                   currB=lastB>=0?b.charAt(lastB):'0';
                   c=currA-'0'+currB-'0'+flag;
                   if (c==3) {
                    chars[i]='1';
                    flag=1;
                }else if (c==2) {
                    chars[i]='0';
                    flag=1;
                }else {
                    chars[i]=(char) ('0'+c);
                    flag=0;
                }
                   lastA--;
                   lastB--;
       }
       String string=new String(chars);
       if (string.charAt(0)=='0') {
        return string.substring(1);
    }
       return string;
    }
}