Merge pull request #822 from AlgorithmWithGod/lkhyun

[20250905] BOJ / G4 / 트리 / 이강현

Author: ShinHeeEul

lkhyun/202509/05 BOJ G4 트리.md

@@ -0,0 +1,72 @@
+```java
+import java.util.*;
+import java.io.*;
+
+public class Main{
+    static BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
+    static BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(System.out));
+    static StringTokenizer st;
+    static StringBuilder sb = new StringBuilder();
+    static int N,M;
+    static int T;
+    public static void main(String[] args) throws Exception {
+        T = 1;
+        while(true){
+            st = new StringTokenizer(br.readLine());
+            N = Integer.parseInt(st.nextToken());
+            M = Integer.parseInt(st.nextToken());
+            if(N == 0 && M == 0) break;
+            sb.append("Case ").append(T++).append(": " );
+
+            int[] parent = new int[N+1];
+            for (int i = 1; i <= N; i++) {
+                parent[i] = i;
+            }
+
+            Set<Integer> cycles = new HashSet<>();
+
+            for (int i = 0; i < M; i++) {
+                st = new StringTokenizer(br.readLine());
+                int a = Integer.parseInt(st.nextToken());
+                int b = Integer.parseInt(st.nextToken());
+                union(a, b, parent, cycles);
+            }
+
+            Set<Integer> s = new HashSet<>();
+            for (int i = 1; i <= N; i++) {
+                s.add(find(i,parent));
+            }
+
+            for(int c : cycles){
+                s.remove(find(c,parent));
+            }
+
+            if(s.size()==0){
+                sb.append("No trees.\n");
+            }else if(s.size()==1){
+                sb.append("There is one tree.\n");
+            }else{
+                sb.append("A forest of ").append(s.size()).append(" trees.\n");
+            }
+        }
+        bw.write(sb.toString());
+        bw.close();
+    }
+    public static int find(int cur, int[] parent){
+        if(parent[cur] == cur) return cur;
+        else return parent[cur] = find(parent[cur],parent);
+    }
+    public static void union(int a, int b, int[] parent, Set<Integer> cycles){
+        int rootA = find(a,parent);
+        int rootB = find(b,parent);
+        
+        if(rootA < rootB){
+            parent[rootA] = rootB;
+        }else if(rootA > rootB){
+            parent[rootB] = rootA;
+        }else{
+            cycles.add(rootA);
+        }
+    }
+}
+```