[20250326] BOJ / G3 / 별자리 만들기 / 신동윤

03do-new30/202503/26 BOJ G3 별자리 만들기.md

@@ -0,0 +1,100 @@
+```java
+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.util.*;
+
+public class Main {
+    static class Point {
+        double r;
+        double c;
+        public Point (double r, double c) {
+            this.r = r;
+            this.c = c;
+        }
+    }
+
+    static class Edge implements Comparable<Edge>{
+        int from;
+        int to;
+        double weight;
+        public Edge(int from, int to, double weight) {
+            this.from = from;
+            this.to = to;
+            this.weight = weight;
+        }
+
+        @Override
+        public int compareTo(Edge o) {
+            return Double.compare(this.weight, o.weight);
+        }
+    }
+    static int n;
+    static int[] parents;
+    static Point[] points;
+    static PriorityQueue<Edge> pq;
+
+
+    public static void main(String[] args) throws IOException {
+        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
+        n = Integer.parseInt(br.readLine());
+        points = new Point[n];
+        for (int i = 0; i < n; i++) {
+            StringTokenizer st = new StringTokenizer(br.readLine());
+            double r = Double.parseDouble(st.nextToken());
+            double c = Double.parseDouble(st.nextToken());
+            points[i] = new Point(r, c);
+        }
+        // 자기 자신을 포함하는 부분집합 구성
+        makeSet();
+        // Point간의 거리를 계산해서 PQ에 등록
+        pq = new PriorityQueue<>();
+        for (int i = 0; i < n-1; i++) {
+            for (int j = i+1; j < n; j++) {
+                pq.offer(new Edge(i, j, getDist(points[i], points[j])));
+            }
+        }
+        // ans: 별자리를 만드는 최소 비용
+        double ans = 0.0;
+        int edgeCnt = 0; // Kruskal 알고리즘은 간선이 n-1개 선택되면 종료
+        while (!pq.isEmpty() && edgeCnt < n-1) {
+            Edge e = pq.poll();
+            if (union(e.from, e.to)) { // from, to가 서로소 집합에 속한다면 ans에 비용 추가
+                ans += e.weight;
+                edgeCnt++;
+            }
+        }
+        System.out.printf("%.2f\n", ans);
+        br.close();
+    }
+
+    private static void makeSet() {
+        parents = new int[n];
+        for (int i = 0; i < n; i++) {
+            parents[i] = i;
+        }
+    }
+
+    private static int find(int a) {
+        if (a == parents[a]) return a;
+        return parents[a] = find(parents[a]);
+    }
+
+    private static boolean union(int a, int b) {
+        int aRoot = find(a);
+        int bRoot = find(b);
+        if (aRoot == bRoot) return false;
+        if (aRoot < bRoot) {
+            parents[bRoot] = aRoot;
+        } else {
+            parents[aRoot] = bRoot;
+        }
+        return true;
+    }
+
+    private static double getDist(Point x, Point y) {
+        return Math.sqrt(Math.pow(x.r - y.r, 2) + Math.pow(x.c - y.c, 2));
+    }
+}
+
+```