Easy 67题 Add Binary

Question:

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) {
        StringBuilder sb=new StringBuilder();
        int flag=0;
        int i=a.length();
        int j=b.length();