230531 소수 찾기

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

public static 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 static 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 static 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