[G4] 4803 트리

java/src/boj4803/Main.java

@@ -0,0 +1,91 @@
+package boj4803;
+
+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.util.HashSet;
+import java.util.Set;
+
+public class Main {
+
+    private static final BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
+    private static int[] parents;
+    private static boolean[] hasCycle;
+
+    public static void main(String[] args) {
+        int t = 1;
+
+        while (true) {
+            String line = readLine();
+            if (line.equals("0 0")) {
+                break;
+            }
+
+            String[] nm = line.split(" ");
+            int n = Integer.parseInt(nm[0]);
+            int m = Integer.parseInt(nm[1]);
+
+            parents = new int[n + 1];
+            hasCycle = new boolean[n + 1];
+            for (int i = 1; i <= n; i++) {
+                parents[i] = i;
+            }
+
+            for (int i = 0; i < m; i++) {
+                String[] elements = readLine().split(" ");
+                int a = Integer.parseInt(elements[0]);
+                int b = Integer.parseInt(elements[1]);
+                if (find(a) == find(b)) {
+                    hasCycle[find(a)] = true;
+                } else {
+                    union(a, b);
+                }
+            }
+
+            Set<Integer> roots = new HashSet<>();
+            int treeCount = 0;
+            for (int i = 1; i <= n; i++) {
+                int root = find(i);
+                if (!roots.contains(root)) {
+                    roots.add(root);
+                    if (!hasCycle[root]) {
+                        treeCount++;
+                    }
+                }
+            }
+
+            if (treeCount == 0) {
+                System.out.println("Case " + t + ": No trees.");
+            } else if (treeCount == 1) {
+                System.out.println("Case " + t + ": There is one tree.");
+            } else {
+                System.out.println("Case " + t + ": A forest of " + treeCount + " trees.");
+            }
+
+            t++;
+        }
+    }
+
+    private static void union(int x, int y) {
+        int rootX = find(x);
+        int rootY = find(y);
+        if (rootX != rootY) {
+            parents[rootY] = rootX;
+        }
+    }
+
+    private static int find(int x) {
+        if (parents[x] != x) {
+            parents[x] = find(parents[x]); // 경로 압축
+        }
+        return parents[x];
+    }
+
+    private static String readLine() {
+        try {
+            return reader.readLine();
+        } catch (IOException e) {
+            throw new RuntimeException(e);
+        }
+    }
+}