[20251217] BOJ / G5 / 신기한 소수 / 강신지

ksinji/202512/17 BOJ 신기한 소수.md

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+```java
+import java.io.*;
+import java.util.*;
+
+public class Main {
+    static int N;
+    static StringBuilder sb = new StringBuilder();
+
+    public static void main(String[] args) throws Exception {
+        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
+        N = Integer.parseInt(br.readLine());
+
+        int[] start = {2, 3, 5, 7};
+        for (int s : start) dfs(s, 1);
+
+        System.out.print(sb);
+    }
+
+    static boolean isPrime(int x) {
+        if (x < 2) return false;
+        if (x == 2) return true;
+        if (x % 2 == 0) return false;
+        for (int i = 3; i * i <= x; i += 2) {
+            if (x % i == 0) return false;
+        }
+        return true;
+    }
+
+    static void dfs(int num, int depth) {
+        if (depth == N) {
+            sb.append(num).append('\n');
+            return;
+        }
+        for (int d = 1; d <= 9; d++) {
+            int next = num * 10 + d;
+            if (isPrime(next)) dfs(next, depth + 1);
+        }
+    }
+}
+```