p67

 
public class  {
    public String addBinary(String a, String b) {
        int al = a.length() - 1;
		int bl = b.length() - 1;
		int c = 0;
		StringBuilder stringBuilder = new StringBuilder();
		while (al >= 0 || bl >= 0) {
			int sum = 0;
			if (al >= 0)
				sum += a.charAt(al--) - '0';
			if (bl >= 0)
				sum += b.charAt(bl--) - '0';
			sum += c;
			stringBuilder.append(sum % 2);
			c = sum / 2;
		}
		if (c == 1) stringBuilder.append(c);
		return stringBuilder.reverse().toString();

    }
}