[20250712] BOJ / G4 / 거짓말 / 신동윤

Author: 03do-new30

03do-new30/202507/12 BOJ G4 거짓말.md

@@ -0,0 +1,94 @@
+```java
+import java.io.*;
+import java.util.*;
+
+public class Main {
+
+    static int N, M;
+    static boolean[] knowingTruth;
+    static int[] parents;
+
+
+    public static void main(String[] args) throws IOException {
+        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
+        StringTokenizer st = new StringTokenizer(br.readLine());
+        N = Integer.parseInt(st.nextToken());
+        M = Integer.parseInt(st.nextToken());
+
+        knowingTruth = new boolean[N+1];
+        Arrays.fill(knowingTruth, false);
+        st = new StringTokenizer(br.readLine());
+        int knowing = Integer.parseInt(st.nextToken());
+        for (int i = 0; i < knowing; i++) {
+            int id = Integer.parseInt(st.nextToken());
+            knowingTruth[id] = true;
+        }
+
+        initParents();
+        List<Integer>[] logs = new List[M];
+        for (int i = 0; i < M; i++) {
+            logs[i] = new ArrayList<>();
+            st = new StringTokenizer(br.readLine());
+            int count = Integer.parseInt(st.nextToken());
+            int[] arr = new int[count];
+            for (int j = 0; j < count; j++) {
+                arr[j] = Integer.parseInt(st.nextToken());
+                logs[i].add(arr[j]);
+            }
+            for (int j = 0; j < count - 1; j++) {
+                for (int k = j + 1; k < count; k++) {
+                    union(arr[j], arr[k]);
+                }
+            }
+        }
+
+        // 거짓말을 할 수 있는 파티인지 판별
+        int answer = 0;
+        for (int i = 0; i < M; i++) {
+            boolean canLie = true;
+            for (int participant : logs[i]) {
+                if (knowingTruth[find(participant)]) {
+                    canLie = false;
+                    break;
+                }
+            }
+            if (canLie) answer++;
+        }
+
+        System.out.println(answer);
+
+
+        br.close();
+    }
+
+    private static void initParents() {
+        parents = new int[N+1];
+        for (int i = 1 ; i < N+1; i++) {
+            parents[i] = i;
+        }
+    }
+
+    private static int find(int x) {
+        if (x == parents[x]) return x;
+        return parents[x] = find(parents[x]);
+    }
+
+    private static boolean union(int a, int b) {
+        int rootA = find(a);
+        int rootB = find(b);
+
+        if (rootA == rootB) return false;
+
+        boolean aKnows = knowingTruth[rootA];
+        boolean bKnows = knowingTruth[rootB];
+
+        if (rootA < rootB) {
+            parents[rootB] = rootA;
+        } else {
+            parents[rootA] = rootB;
+        }
+        knowingTruth[rootA] = knowingTruth[rootB] = aKnows || bKnows;
+        return true;
+    }
+}
+```