[20250714] BOJ / P3 / 로버트 후드 / 권혁준

khj20006/202507/[BOJ-9240] 로버트 후드.md

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+```java
+import java.util.*;
+import java.io.*;
+
+class IOController {
+    BufferedReader br;
+    BufferedWriter bw;
+    StringTokenizer st;
+
+    public IOController() {
+        br = new BufferedReader(new InputStreamReader(System.in));
+        bw = new BufferedWriter(new OutputStreamWriter(System.out));
+        st = new StringTokenizer("");
+    }
+
+    String nextLine() throws Exception {
+        String line = br.readLine();
+        st = new StringTokenizer(line);
+        return line;
+    }
+
+    String nextToken() throws Exception {
+        while (!st.hasMoreTokens()) nextLine();
+        return st.nextToken();
+    }
+
+    int nextInt() throws Exception {
+        return Integer.parseInt(nextToken());
+    }
+
+    long nextLong() throws Exception {
+        return Long.parseLong(nextToken());
+    }
+
+    double nextDouble() throws Exception {
+        return Double.parseDouble(nextToken());
+    }
+
+    void close() throws Exception {
+        bw.flush();
+        bw.close();
+    }
+
+    void write(String content) throws Exception {
+        bw.write(content);
+    }
+
+}
+
+class Point {
+    int x, y;
+    Point(int x, int y) {
+        this.x = x;
+        this.y = y;
+    }
+
+    int ccw(Point p1, Point p2) {
+        int res = p1.x*p2.y + p2.x*this.y + this.x*p1.y - (p2.x*p1.y + this.x*p2.y + p1.x*this.y);
+        if(res < 0) return -1;
+        if(res > 0) return 1;
+        return 0;
+    }
+
+    double dist(Point o) {
+        return Math.sqrt((this.x - o.x) * (this.x - o.x) + (this.y - o.y) * (this.y - o.y));
+    }
+}
+
+public class Main {
+
+    static IOController io;
+
+    //
+
+    static int N;
+    static Point[] a;
+
+    public static void main(String[] args) throws Exception {
+
+        io = new IOController();
+
+        init();
+        solve();
+
+        io.close();
+    }
+
+    public static void init() throws Exception {
+
+        N = io.nextInt();
+        a = new Point[N];
+        for(int i=0;i<N;i++) a[i] = new Point(io.nextInt(),io.nextInt());
+
+    }
+
+    static void solve() throws Exception {
+
+        // 볼록 껍질 만들기
+        Arrays.sort(a, (a,b) -> a.x==b.x ? a.y-b.y : a.x-b.x);
+        List<Point> lower = new ArrayList<>();
+        List<Point> upper = new ArrayList<>();
+        for(Point p:a) {
+            while(lower.size() > 1 && p.ccw(lower.get(lower.size()-2), lower.get(lower.size()-1)) <= 0) lower.remove(lower.size()-1);
+            lower.add(p);
+            while(upper.size() > 1 && p.ccw(upper.get(upper.size()-2), upper.get(upper.size()-1)) >= 0) upper.remove(upper.size()-1);
+            upper.add(p);
+        }
+
+        List<Point> hull = new ArrayList<>();
+        for(int i=0;i<lower.size()-1;i++) hull.add(lower.get(i));
+        for(int i=upper.size()-1;i>0;i--) hull.add(upper.get(i));
+
+
+
+        // 회전하는 캘리퍼스 구현
+        double ans = 0;
+        N = hull.size();
+
+        int j = 1;
+        for(int i=0;i<N;i++) {
+            Point p1 = hull.get(i);
+            Point p2 = hull.get((i+1)%N);
+            Point p3 = hull.get(j);
+            Point p4 = hull.get((j+1)%N);
+            Point p5 = new Point(p4.x - (p3.x - p2.x), p4.y - (p3.y - p2.y));
+            while(p5.ccw(p1, p2) > 0) {
+                j = (j+1)%N;
+                p3 = hull.get(j);
+                p4 = hull.get((j+1)%N);
+                p5 = new Point(p4.x - (p3.x - p2.x), p4.y - (p3.y - p2.y));
+            }
+            ans = Math.max(ans, p1.dist(p3));
+        }
+        io.write(ans + "\n");
+
+    }
+
+}
+```