240130 소수 찾기

char[] chs;
boolean[] visited;
HashSet<Integer> set;

public int solution(String numbers) {
	int len = numbers.length();
	visited = new boolean[len];
	set = new HashSet<>();

	for (int i = 1; i <= len; i++) {
		chs = new char[i];
		permutation(0, i, len, numbers);
	}

	return set.size();
}

public void permutation(int start, int r, int n, String numbers) {
	if (start == r) {
		if (chs[0] == '0')
			return;
		int num = Integer.parseInt(String.valueOf(chs));
		if (isPrimeNumber(num)) {
			set.add(num);
		}
		return;
	}

	for (int i = 0; i < n; i++) {
		if (visited[i])
			continue;

		visited[i] = true;
		chs[start] = numbers.charAt(i);
		permutation(start + 1, r, n, numbers);
		visited[i] = false;
	}
}

public boolean isPrimeNumber(int num) {
	if (num == 1)
		return false;

	for (int i = 2; i <= Math.sqrt(num); i++) {
		if (num % i == 0)
			return false;
	}
	return true;
}

출처:https://school.programmers.co.kr/learn/courses/30/lessons/42839