Solution.java

class Solution {
    public String addBinary(String a, String b) {
        StringBuilder sb = new StringBuilder();
        for (int i = a.length() - 1, j = b.length() - 1, carry = 0; i >= 0 || j >= 0 || carry > 0;
             --i, --j) {
            carry += (i >= 0 ? a.charAt(i) - '0' : 0) + (j >= 0 ? b.charAt(j) - '0' : 0);
            sb.append(carry % 2);
            carry /= 2;
        }
        return sb.reverse().toString();
    }
}