[20250203] BOJ / P5 / LCA2 / 신희을

ShinHeeEul/202502/03 BOJ P5 LCA2 신희을.md

@@ -0,0 +1,140 @@
+```java
+import java.io.*;
+import java.util.*;
+
+public class Solution {
+
+	static List<Integer>[] lists;
+	static int[][] parents;
+	static int[] depths;
+	static int N;
+	static Queue<Integer> update = new LinkedList<>();
+
+	public static void main(String[] args) throws Exception{
+		// LCA
+		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
+
+	// 전처리
+
+		N = Integer.parseInt(br.readLine());
+		lists = new ArrayList[N + 1];
+		parents = new int[N + 1][17];
+		depths = new int[N + 1];
+
+		for(int i = 0; i < N + 1; i++) {
+			lists[i] = new ArrayList<>();
+		}
+
+		// 내려가면서 tree 만들어
+		for(int i = 1; i < N; i++) {
+			StringTokenizer st = new StringTokenizer(br.readLine());
+			int a = Integer.parseInt(st.nextToken());
+			int b = Integer.parseInt(st.nextToken());
+			lists[a].add(b);
+			lists[b].add(a);
+		}
+
+		//자식 트리 생성
+		dfs();
+		// 2^k Parent 배열 만들어
+			// 2^k+1 번째 부모는 2^k번째 부모의 2^k번째 부모
+			// parent[i][j + 1] = parent[parent[i][j]][j];
+
+		while(!update.isEmpty()) {
+			int i = update.poll();
+			for(int j = 0; j < 16; j++) parents[i][j + 1] = parents[parents[i][j]][j];
+		}
+
+
+		int M = Integer.parseInt(br.readLine());
+
+		StringBuilder sb = new StringBuilder();
+		for(int m = 0; m < M; m++) {
+			StringTokenizer st = new StringTokenizer(br.readLine());
+
+			int a = Integer.parseInt(st.nextToken());
+			int b = Integer.parseInt(st.nextToken());
+
+			// 뎊스 차이만큼 올라가 (뎊스를 맞춰)
+			int depthA = depths[a];
+			int depthB = depths[b];
+
+			if(depthA > depthB) {
+				a = calculateDiff(a, depthA - depthB);
+			} else {
+				b = calculateDiff(b, depthB - depthA);
+			}
+
+			// 2^k 만큼 올라가면서 부모 찾아
+				// 일치 하지 않아? 또 올라가
+				// 일치해? 내려가
+
+			for(int i = 16; i >= 0; i--) {
+				int pa = parents[a][i];
+				int pb = parents[b][i];
+
+				if(pa != pb) {
+					a = pa;
+					b = pb;
+					i++;
+				}
+			}
+			if(a == b) sb.append(a).append("\n");
+			else sb.append(parents[a][0]).append("\n");
+		}
+
+		System.out.println(sb);
+
+
+	}
+
+	public static int calculateDiff(int val, int diff) {
+		for(int i = 0; diff >= (1 << i); i++) {
+			int binary = 1 << i;
+			if((binary | diff) == diff) {
+				val = parents[val][i];
+			}
+		}
+
+		return val;
+	}
+
+	public static void dfs() {
+		Stack<Node> stack = new Stack<>();
+		boolean[] visited = new boolean[N + 1];
+
+		stack.add(new Node(1, 1));
+		visited[1] = true;
+		update.add(1);
+
+		while(!stack.isEmpty()) {
+			Node node = stack.pop();
+
+			int parent = node.val;
+			int depth = node.depth;
+			depths[parent] = depth;
+			update.add(parent);
+
+			for(int a : lists[parent]) {
+				if(visited[a]) {
+					parents[parent][0] = a;
+					continue;
+				}
+				visited[a] = true;
+				stack.push(new Node(a, depth + 1));
+			}
+		}
+	}
+
+	public static class Node {
+		int val;
+		int depth;
+
+		public Node(int val, int depth) {
+			this.val = val;
+			this.depth = depth;
+		}
+	}
+
+}
+```