67. Add Binary

class Solution {
    public String addBinary(String a, String b) {
        int i = a.length() - 1, j = b.length() - 1;
        StringBuilder sb = new StringBuilder();
        int carry = 0;
        while (i >= 0 || j >= 0 || carry > 0) {
            int x = i >= 0 ? a.charAt(i--) - '0' : 0;
            int y = j >= 0 ? b.charAt(j--) - '0' : 0;
            int sum = carry + x + y;
            sb.append(sum & 1);
            carry = sum >> 1;
        }

        return sb.reverse().toString();
    }
}

Reference

67. Add Binary
剑指 Offer II 002. 二进制加法