Merge pull request #1223 from AlgorithmWithGod/0224LJH

[20251025] PGM / Lv3 / 등대 / 이종환

Author: ShinHeeEul

0224LJH/202510/25 PGM 등대.md

@@ -0,0 +1,79 @@
+```java
+import java.io.*;
+import java.util.*;
+
+class Solution {
+    static int nodeCnt, answer;
+    static Node[] nodes;
+    
+    static class Node{
+        int num;
+        boolean isOn = false;
+        Node parent = null;
+        HashSet<Node> to = new HashSet<>();
+        
+        public Node(int num){
+            this.num = num;
+        }
+    }
+    
+    public int solution(int n, int[][] lighthouse) {
+        nodeCnt = n;
+        answer = 0;
+        init(lighthouse);
+        
+        makeTree(1);
+        turnLight(1);
+        for (int i = 1; i<= nodeCnt; i++){
+            if (nodes[i].isOn)answer++;
+        }
+        answer = Math.min(answer, nodeCnt - answer);
+        
+        return answer;
+    }
+    
+    private void turnLight(int nodeNum){
+        Node node = nodes[nodeNum];
+        if (node.to.size() == 0){
+            node.isOn = false;
+            return;
+        }
+        
+        boolean result = true;
+        for (Node child: node.to){
+            turnLight(child.num);
+            result &= child.isOn;
+        }
+        node.isOn = !result;
+    }
+    
+    private void makeTree(int nodeNum){
+        Node node = nodes[nodeNum];
+        for (Node child: node.to){
+            child.parent = node;
+            child.to.remove(node);
+            makeTree(child.num);
+        }
+        
+    }
+    
+    private void init(int[][] lighthouse){
+        
+        nodes = new Node[nodeCnt+1];
+        for (int i = 0; i <= nodeCnt; i++){
+            nodes[i] = new Node(i);
+        }
+        
+        for (int i = 0; i< nodeCnt-1; i++){
+            int n1 = lighthouse[i][0];
+            int n2 = lighthouse[i][1];
+            
+            Node node1 = nodes[n1];
+            Node node2 = nodes[n2];
+            node1.to.add(node2);
+            node2.to.add(node1);
+            
+        }
+    }
+}
+```