[20250921] BOJ / G4 / 가장 가까운 공통 조상 / 이종환

Author: 0224LJH

0224LJH/202509/21 BOJ 가장 가까운 공통 조상.md

@@ -0,0 +1,91 @@
+```java
+
+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.util.*;
+
+class Main {
+	static BufferedReader br= new BufferedReader(new InputStreamReader(System.in));
+	static int nodeCnt, num1, num2,ans;
+	static Node[] nodes;
+	static HashSet<Node> set = new HashSet<>();
+	
+	static class Node{
+		int num;
+		Node parent;
+		HashSet<Node> children = new HashSet<>();
+
+	}
+	
+    
+    public static void main(String[] args) throws NumberFormatException, IOException {
+		int TC = Integer.parseInt(br.readLine());
+		for (int tc = 1; tc <= TC; tc++) {
+	        init();
+	    	process();
+	    	print();
+
+		}
+
+    }
+
+    
+    public static void init() throws NumberFormatException, IOException {
+    	
+    	nodeCnt = Integer.parseInt(br.readLine());
+    	nodes = new Node[nodeCnt+1];
+    	
+    	for (int i = 1; i <= nodeCnt; i++) {
+    		nodes[i] = new Node();
+    		nodes[i].num = i;
+    	}
+    	
+    	for (int i = 0; i < nodeCnt-1; i++) {
+    		StringTokenizer st = new StringTokenizer(br.readLine());
+    		int parentNum = Integer.parseInt(st.nextToken());
+    		int childNum = Integer.parseInt(st.nextToken());
+    		
+    		nodes[parentNum].children.add(nodes[childNum]);
+    		nodes[childNum].parent = nodes[parentNum];
+    	}
+    	
+    	StringTokenizer st = new StringTokenizer(br.readLine());
+    	num1 = Integer.parseInt(st.nextToken());
+    	num2 = Integer.parseInt(st.nextToken());
+    	
+    	
+
+    }
+    
+    public static void process() { 	
+    	set.clear();
+    	
+    	Node cur = nodes[num1];
+    	set.add(cur);
+    	while(cur.parent != null) {
+    		set.add(cur);
+    		cur = cur.parent;
+    	}
+    	
+    	cur = nodes[num2];
+    	while(cur.parent != null) {
+    		if (set.contains(cur)) {
+    			ans = cur.num;
+    			return;
+    		}
+    		cur = cur.parent;
+    	}
+    	
+    	ans = cur.num;
+    	
+    }
+
+
+
+
+	public static void print() {
+		System.out.println(ans);
+    }
+}
+```