[20251204] BOJ / G3 / 선물 교환 / 이종환

0224LJH/202512/04 BOJ 선물 교환.md

@@ -0,0 +1,103 @@
+```java
+
+import java.util.StringTokenizer;
+import java.util.*;
+import java.io.*;
+
+public class Main {
+
+   static BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
+   static int size;
+   static Student[] students;
+   static List<Integer> ans = new ArrayList<>();
+   static HashSet<Student> set = new HashSet<>();
+
+   static StringBuilder sb = new StringBuilder();
+
+
+   static class Student implements Comparable<Student>{
+	   int presentCnt = 0;
+	   int idx;
+	   int target1;
+	   int target2;
+
+	   public Student (int idx,int t1, int t2) {
+		   this.idx = idx;
+		   target1 = t1;
+		   target2 = t2;
+	   }
+
+	   @Override
+	   public int compareTo(Student s) {
+		   return Integer.compare(this.presentCnt, s.presentCnt);
+	   }
+   }
+
+    public static void main(String[] args) throws IOException {
+    	init();
+    	process();
+    	print();
+
+    }
+
+    private static void init() throws IOException{
+    	size = Integer.parseInt(br.readLine());
+    	students = new Student[size+1];
+
+    	for (int i = 1; i <= size; i++) {
+    		StringTokenizer st = new StringTokenizer(br.readLine());
+    		int t1 = Integer.parseInt(st.nextToken());
+    		int t2 = Integer.parseInt(st.nextToken());
+    		students[i] = new Student(i,t1, t2);
+    		set.add(students[i]);
+    	}
+
+    	for (int i = 1; i <= size; i++) {
+    		Student s = students[i];
+    		students[s.target1].presentCnt++;
+    		students[s.target2].presentCnt++;
+    	}
+    }
+
+    private static void process() throws IOException {
+    	PriorityQueue<Student> pq = new PriorityQueue<>();
+
+    	for (int i = 1; i <= size; i++) {
+    		pq.add(students[i]);
+    	}
+
+    	while(pq.size() > 0 &&  pq.peek().presentCnt < 2) {
+    		Student s = pq.poll();
+    		if (!set.contains(s)) continue;
+    		set.remove(s);
+    		students[s.target1].presentCnt--;
+    		students[s.target2].presentCnt--;
+
+    		if (students[s.target1].presentCnt < 2) {
+    			pq.add(students[s.target1]);
+    		}
+    		if (students[s.target2].presentCnt < 2) {
+    			pq.add(students[s.target2]);
+    		}
+    	}
+
+    	for (int i = 1; i <= size; i++) {
+    		if ( students[i].presentCnt == 2) ans.add(i);
+    	}
+
+    	sb.append(ans.size());
+    	if (ans.size() == 0) return;
+    	sb.append("\n");
+    	for (int n: ans) {
+    		sb.append(n).append(" ");
+    	}
+
+
+    }
+
+   private static void print() {
+      System.out.println(sb);
+    }
+
+}
+```