Merge pull request #412 from AlgorithmWithGod/khj20006

[20250706] BOJ / P4 / 가톨릭대학교에 워터 슬라이드를?? / 권혁준

Author: oncsr

khj20006/202507/06 BOJ P4 가톨릭대학교에 워터 슬라이드를??.md

@@ -0,0 +1,140 @@
+```java
+import java.util.*;
+import java.io.*;
+
+class IOController {
+    BufferedReader br;
+    BufferedWriter bw;
+    StringTokenizer st;
+
+    public IOController() {
+        br = new BufferedReader(new InputStreamReader(System.in));
+        bw = new BufferedWriter(new OutputStreamWriter(System.out));
+        st = new StringTokenizer("");
+    }
+
+    String nextLine() throws Exception {
+        String line = br.readLine();
+        st = new StringTokenizer(line);
+        return line;
+    }
+
+    String nextToken() throws Exception {
+        while (!st.hasMoreTokens()) nextLine();
+        return st.nextToken();
+    }
+
+    int nextInt() throws Exception {
+        return Integer.parseInt(nextToken());
+    }
+
+    long nextLong() throws Exception {
+        return Long.parseLong(nextToken());
+    }
+
+    double nextDouble() throws Exception {
+        return Double.parseDouble(nextToken());
+    }
+
+    void close() throws Exception {
+        bw.flush();
+        bw.close();
+    }
+
+    void write(String content) throws Exception {
+        bw.write(content);
+    }
+
+}
+
+public class Main {
+
+    static IOController io;
+
+    //
+
+    static int N, M;
+    static List<Integer>[] G, R;
+    static Stack<Integer> S;
+    static int[] C, D;
+    static boolean[] vis;
+
+    public static void main(String[] args) throws Exception {
+
+        io = new IOController();
+
+        init();
+        solve();
+
+        io.close();
+
+    }
+
+    public static void init() throws Exception {
+
+        N = io.nextInt();
+        M = io.nextInt();
+        G = new List[N+1];
+        R = new List[N+1];
+        for(int i=1;i<=N;i++) {
+            G[i] = new ArrayList<>();
+            R[i] = new ArrayList<>();
+        }
+
+        while(M-- > 0) {
+            int a = io.nextInt(), b = io.nextInt();
+            G[a].add(b);
+            R[b].add(a);
+        }
+
+        S = new Stack<>();
+        C = new int[N+1];
+
+    }
+
+    static void solve() throws Exception {
+
+        vis = new boolean[N+1];
+        for(int i=1;i<=N;i++) if(!vis[i]) {
+            vis[i] = true;
+            dfs1(i);
+        }
+
+        vis = new boolean[N+1];
+        int cnt = 0;
+        while(!S.isEmpty()) {
+            int n = S.pop();
+            if(vis[n]) continue;
+            vis[n] = true;
+            dfs2(n,++cnt);
+        }
+
+        D = new int[cnt+1];
+        for(int i=1;i<=N;i++) for(int j:G[i]) {
+            if(C[i] != C[j]) D[C[j]]++;
+        }
+
+        int ans = 0;
+        for(int i=1;i<=cnt;i++) if(D[i] == 0) ans++;
+        io.write(ans + "\n");
+
+    }
+
+    static void dfs1(int n) {
+        for(int i:G[n]) if(!vis[i]) {
+            vis[i] = true;
+            dfs1(i);
+        }
+        S.add(n);
+    }
+
+    static void dfs2(int n, int c) {
+        C[n] = c;
+        for(int i:R[n]) if(!vis[i]) {
+            vis[i] = true;
+            dfs2(i,c);
+        }
+    }
+
+}
+```