231214 브라이언의 고민

private static final String INVALID = "invalid";

private String temp;

public String solution(String sentence) {
	boolean[] used = new boolean[26];
	StringBuilder answer = new StringBuilder();

	temp = sentence;
	while (!temp.isEmpty()) {
		String ret;
		List<Integer> pos = new ArrayList<>();

		if (Character.isLowerCase(temp.charAt(0))) {
			ret = processLowerCase(used, pos);
		} else {
			ret = processUpperCase(used, pos);
		}

		if (ret.equals(INVALID)) {
			return INVALID;
		}

		answer.append(ret).append(" ");
	}

	answer.delete(answer.length() - 1, answer.length());
	return answer.toString();
}

private String processLowerCase(boolean[] used, List<Integer> pos) {
	if (used[temp.charAt(0) - 'a']) {
		return INVALID;
	}
	used[temp.charAt(0) - 'a'] = true;

	for (int i = 0; i < temp.length(); i++) {
		if (temp.charAt(i) == temp.charAt(0)) {
			pos.add(i);
		}
	}

	if (pos.size() != 2) {
		return INVALID;
	}

	String center = temp.substring(1, pos.get(pos.size() - 1));
	if (center.isEmpty()) {
		return INVALID;
	}

	String ret = rule1(center);
	if (ret.equals("-1")) {
		ret = allUpper(center);
		if (ret.equals("-1")) {
			return INVALID;
		}
	} else {
		if (used[temp.charAt(2) - 'a']) {
			return INVALID;
		}
		used[temp.charAt(2) - 'a'] = true;
	}

	temp = temp.substring(pos.get(pos.size() - 1) + 1);
	return ret;
}

private String processUpperCase(boolean[] used, List<Integer> pos) {
	String ret;
	if (temp.length() == 1 || Character.isUpperCase(temp.charAt(1))) {
		ret = String.valueOf(temp.charAt(0));
		temp = temp.substring(1);
	} else {
		for (int i = 0; i < temp.length(); i++) {
			if (temp.charAt(1) == temp.charAt(i)) {
				pos.add(i);
			}
		}

		if (pos.size() != 2) {
			if (pos.get(pos.size() - 1) == temp.length() - 1) {
				return INVALID;
			}
			if (Character.isLowerCase(temp.charAt(pos.get(pos.size() - 1) + 1))) {
				return INVALID;
			}

			String center = temp.substring(0, pos.get(pos.size() - 1) + 2);
			ret = rule1(center);
			if (ret.equals("-1")) {
				return INVALID;
			}

			if (used[temp.charAt(1) - 'a']) {
				return INVALID;
			}
			used[temp.charAt(1) - 'a'] = true;

			temp = temp.substring(pos.get(pos.size() - 1) + 2);
		} else {
			ret = temp.charAt(0) + "";
			temp = temp.substring(1);
		}
	}
	return ret;
}

private String rule1(String str) {
	StringBuilder ret = new StringBuilder();
	if (str.length() < 3) {
		return "-1";
	}
	char c = str.charAt(1);
	boolean ok = false;

	for (int i = 0; i < str.length(); i++) {
		if (Character.isLowerCase(str.charAt(i))) {
			ok = true;
		}
		if (i % 2 == 0) {
			if (Character.isLowerCase(str.charAt(i))) {
				return "-1";
			}
			ret.append(str.charAt(i));
		} else {
			if (c != str.charAt(i)) {
				return "-1";
			}
		}
	}

	if (!ok) {
		return "-1";
	}
	return ret.toString();
}

private String allUpper(String str) {
	StringBuilder ret = new StringBuilder();
	for (char c : str.toCharArray()) {
		if (!Character.isUpperCase(c)) {
			return "-1";
		}
		ret.append(c);
	}
	return ret.toString();
}

출처:https://school.programmers.co.kr/learn/courses/30/lessons/1830