[LeetCode] Add Binary

public static String addBinary(String a, String b) {
		if (null == a && null == b) {
			return null;
		}

		if (null == a) {
			return b;
		}

		if (null == b) {
			return a;
		}

		int carry = 0;

		int p1 = a.length() - 1;
		int p2 = b.length() - 1;

		StringBuilder result = new StringBuilder();

		while (p1 >= 0 || p2 >= 0) {
			int sum = carry;
			if (p1 >= 0) {
				sum += (a.charAt(p1) - ‘0‘);
			}
			if (p2 >= 0) {
				sum += (b.charAt(p2) - ‘0‘);
			}
			char c = sum % 2 == 1 ? ‘1‘ : ‘0‘;
			result.insert(0, c);
			carry = sum / 2;
			p1--;
			p2--;

		}

		if (1 == carry) {
			result.insert(0, "1");
		}

		return result.toString();
	}