[20251027] BOJ / G5 / 숫자고르기 / 설진영

Author: Seol-JY

Seol-JY/202510/27 BOJ 숫자고르기.md

@@ -0,0 +1,74 @@
+```java
+import java.io.*;
+import java.util.*;
+
+public class Main {
+    static int N;
+    static int[] next;
+    static boolean[] finished;
+    static boolean[] inCycle;
+    
+    public static void main(String[] args) throws IOException {
+        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
+        BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(System.out));
+        
+        N = Integer.parseInt(br.readLine());
+        next = new int[N + 1];
+        finished = new boolean[N + 1];
+        inCycle = new boolean[N + 1];
+        
+        for (int i = 1; i <= N; i++) {
+            next[i] = Integer.parseInt(br.readLine());
+        }
+        
+        for (int i = 1; i <= N; i++) {
+            if (!finished[i]) {
+                boolean[] visited = new boolean[N + 1];
+                int current = i;
+                List<Integer> path = new ArrayList<>();
+                
+                while (!visited[current] && !finished[current]) {
+                    visited[current] = true;
+                    path.add(current);
+                    current = next[current];
+                }
+                
+                if (!finished[current]) {
+                    int cycleStart = current;
+                    boolean inCycleNow = false;
+                    
+                    for (int node : path) {
+                        if (node == cycleStart) {
+                            inCycleNow = true;
+                        }
+                        if (inCycleNow) {
+                            inCycle[node] = true;
+                        }
+                        finished[node] = true;
+                    }
+                } else {
+                    for (int node : path) {
+                        finished[node] = true;
+                    }
+                }
+            }
+        }
+        
+        List<Integer> result = new ArrayList<>();
+        for (int i = 1; i <= N; i++) {
+            if (inCycle[i]) {
+                result.add(i);
+            }
+        }
+        
+        bw.write(result.size() + "\n");
+        for (int num : result) {
+            bw.write(num + "\n");
+        }
+        
+        bw.flush();
+        bw.close();
+        br.close();
+    }
+}
+```