Merge pull request #1997 from AlgorithmWithGod/Ukj0ng

[20260306] BOJ / G3 / LCA / 한종욱

Author: ShinHeeEul

Ukj0ng/202603/06 BOJ G3 LCA.md

@@ -0,0 +1,95 @@
+```
+import java.io.*;
+
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Queue;
+import java.util.StringTokenizer;
+
+public class Main {
+    private static final BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
+    private static final BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(System.out));
+    private static List<Integer>[] graph;
+    private static int[][] depth;
+    private static boolean[] visited;
+    private static int N, M;
+
+    public static void main(String[] args) throws IOException {
+        init();
+        bw.flush();
+        bw.close();
+        br.close();
+    }
+
+    private static void init() throws IOException {
+        N = Integer.parseInt(br.readLine());
+
+        graph = new List[N+1];
+        depth = new int[N+1][2];
+        visited = new boolean[N+1];
+
+        for (int i = 1; i <= N; i++) {
+            graph[i] = new ArrayList<>();
+        }
+
+        for (int i = 0; i < N-1; i++) {
+            StringTokenizer st = new StringTokenizer(br.readLine());
+            int node1 = Integer.parseInt(st.nextToken());
+            int node2 = Integer.parseInt(st.nextToken());
+
+            graph[node1].add(node2);
+            graph[node2].add(node1);
+        }
+
+        BFS();
+
+        M = Integer.parseInt(br.readLine());
+        for (int i = 0; i < M; i++) {
+            StringTokenizer st = new StringTokenizer(br.readLine());
+            int node1 = Integer.parseInt(st.nextToken());
+            int node2 = Integer.parseInt(st.nextToken());
+
+            int depth1 = depth[node1][0];
+            int depth2 = depth[node2][0];
+
+            while (depth1 != depth2) {
+                if (depth1 > depth2) {
+                    node1 = depth[node1][1];
+                    depth1 = depth[node1][0];
+                } else {
+                    node2 = depth[node2][1];
+                    depth2 = depth[node2][0];
+                }
+            }
+
+            while (node1 != node2) {
+                node1 = depth[node1][1];
+                node2 = depth[node2][1];
+            }
+
+            bw.write(node1 + "\n");
+        }
+    }
+
+    private static void BFS() {
+        Queue<int[]> q = new ArrayDeque<>();
+        depth[1][0] = 1;
+        depth[1][1] = -1;
+        visited[1] = true;
+        q.add(new int[] {1, 1});
+
+        while (!q.isEmpty()) {
+            int[] current = q.poll();
+
+            for (int next : graph[current[0]]) {
+                if (visited[next]) continue;
+                depth[next][0] = current[1]+1;
+                depth[next][1] = current[0];
+                visited[next] = true;
+                q.add(new int[]{next, current[1]+1});
+            }
+        }
+    }
+}
+```