TopCoder SRM 144 DIV 1 300-points

BinaryCode

class BinaryCode {
	int charToNum(char c) {
		return c - '0';
	}
	
	String solve(String message, char[] P, char first) {
		boolean isValid = true;
		int N = P.length;
		P[0] = first;
		P[1] = (char) ('0' + charToNum(message.charAt(0)) - charToNum(P[0]));
		if (P[1] == '0' || P[1] == '1') {
			for (int i = 1; i < N-1; ++i) {
				P[i+1] = (char) ('0' + charToNum(message.charAt(i)) - charToNum(P[i]) - charToNum(P[i-1]));
				if (P[i+1] != '0' && P[i+1] != '1') {
					isValid = false;
					break;
				}
			}
			if (isValid)
				return new String(P);
			else
				return "NONE";
		}
		else
			return "NONE";
	}
	
	public String[] decode(String message) {
		String[] res = new String[2];
		int N = message.length();
		if (N == 1) {
			char c = message.charAt(0);
			if (c != '0' && c != '1') {
				res[0] = "NONE";
				res[1] = "NONE";
			}
			else {
				if (c == '0') {
					res[0] = new String(message);
					res[1] = "NONE";
				}
				else {
					res[0] = "NONE";
					res[1] = new String(message);
				}
			}
			return res;
		}
		char P[] = new char[N];
		res[0] = solve(message, P, '0');
		res[1] = solve(message, P, '1');
		return res;
	}
}