241129 단축키 지정

public static void main(String[] args) throws IOException {
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
	int n = Integer.parseInt(br.readLine());
	boolean[] isCheck = new boolean[26];

	StringBuilder sb = new StringBuilder();

	for (int t = 0; t < n; t++) {
		String option = br.readLine();
		String str = findRepresentativeKey(option, isCheck);

		sb.append(str).append("\n");
	}

	System.out.println(sb);
}

static String findRepresentativeKey(String option, boolean[] isCheck) {
	for (int i = 0; i < option.length(); i++) {
		char alphabet = option.charAt(i);

		if (alphabet >= 'A' && alphabet <= 'Z') {
			alphabet = (char) ('a' + (alphabet - 'A'));
		}
		int num = alphabet - 'a';

		if (!isCheck[num]) {
			isCheck[num] = true;
			return makeKey(option, i);
		}

		while (true) {
			i++;
			if (i >= option.length() || option.charAt(i) == ' ')
				break;
		}
	}

	for (int i = 1; i < option.length(); i++) {
		if (option.charAt(i) == ' ') {
			i++;
			continue;
		}

		char alphabet = option.charAt(i);

		if (alphabet >= 'A' && alphabet <= 'Z') {
			alphabet = (char) ('a' + (alphabet - 'A'));
		}
		int num = alphabet - 'a';

		if (!isCheck[num]) {
			isCheck[num] = true;
			return makeKey(option, i);
		}
	}

	return option;
}

static String makeKey(String option, int idx) {
	StringBuilder sb = new StringBuilder();
	sb.append(option, 0, idx);
	sb.append("[").append(option.charAt(idx)).append("]");
	sb.append(option.substring(idx + 1));

	return sb.toString();
}

출처:https://www.acmicpc.net/problem/1283