[20251026] BOJ / G1 / 다리 만들기 2 / 이준희

Author: JHLEE325

JHLEE325/202510/26 BOJ G1 다리 만들기 2.md

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+```java
+import java.io.*;
+import java.util.*;
+
+public class Main {
+    static int n, m;
+    static int[][] map;
+    static boolean[][] visited;
+    static int[] dx = {-1, 1, 0, 0};
+    static int[] dy = {0, 0, -1, 1};
+    static int islandnum = 0;
+    static List<Edge> edges = new ArrayList<>();
+
+    static class Edge implements Comparable<Edge> {
+        int from, to, dist;
+        Edge(int f, int t, int d) {
+            from = f; to = t; dist = d;
+        }
+        public int compareTo(Edge o) {
+            return this.dist - o.dist;
+        }
+    }
+
+    static int[] parent;
+
+    public static void main(String[] args) throws IOException {
+        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
+        StringTokenizer st = new StringTokenizer(br.readLine());
+        n = Integer.parseInt(st.nextToken());
+        m = Integer.parseInt(st.nextToken());
+
+        map = new int[n][m];
+        visited = new boolean[n][m];
+
+        for (int i = 0; i < n; i++) {
+            st = new StringTokenizer(br.readLine());
+            for (int j = 0; j < m; j++) {
+                map[i][j] = Integer.parseInt(st.nextToken());
+            }
+        }
+
+        int id = 2;
+        for (int i = 0; i < n; i++) {
+            for (int j = 0; j < m; j++) {
+                if (map[i][j] == 1 && !visited[i][j]) {
+                    findisland(i, j, id);
+                    id++;
+                }
+            }
+        }
+        islandnum = id - 2;
+
+        bridge();
+
+        parent = new int[islandnum + 2];
+        for (int i = 0; i <= islandnum + 1; i++) parent[i] = i;
+        Collections.sort(edges);
+
+        int used = 0;
+        int total = 0;
+
+        for (Edge e : edges) {
+            if (union(e.from, e.to)) {
+                total += e.dist;
+                used++;
+                if (used == islandnum - 1) break;
+            }
+        }
+
+        if (used == islandnum - 1)
+            System.out.println(total);
+        else
+            System.out.println(-1);
+    }
+
+    static void findisland(int x, int y, int id) {
+        Queue<int[]> q = new LinkedList<>();
+        q.add(new int[]{x, y});
+        visited[x][y] = true;
+        map[x][y] = id;
+
+        while (!q.isEmpty()) {
+            int[] now = q.poll();
+            for (int d = 0; d < 4; d++) {
+                int nx = now[0] + dx[d];
+                int ny = now[1] + dy[d];
+                if (nx < 0 || ny < 0 || nx >= n || ny >= m) continue;
+                if (map[nx][ny] == 1 && !visited[nx][ny]) {
+                    visited[nx][ny] = true;
+                    map[nx][ny] = id;
+                    q.add(new int[]{nx, ny});
+                }
+            }
+        }
+    }
+
+    static void bridge() {
+        for (int i = 0; i < n; i++) {
+            for (int j = 0; j < m; j++) {
+                if (map[i][j] >= 2) {
+                    int from = map[i][j];
+                    for (int d = 0; d < 4; d++) {
+                        int nx = i;
+                        int ny = j;
+                        int dist = 0;
+
+                        while (true) {
+                            nx += dx[d];
+                            ny += dy[d];
+                            if (nx < 0 || ny < 0 || nx >= n || ny >= m) break;
+                            if (map[nx][ny] == from) break;
+                            if (map[nx][ny] == 0) {
+                                dist++;
+                                continue;
+                            }
+                            if (map[nx][ny] >= 2) {
+                                if (dist >= 2) {
+                                    edges.add(new Edge(from, map[nx][ny], dist));
+                                }
+                                break;
+                            }
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    static int find(int x) {
+        if (x == parent[x]) return x;
+        return parent[x] = find(parent[x]);
+    }
+
+    static boolean union(int a, int b) {
+        a = find(a);
+        b = find(b);
+        if (a == b) return false;
+        parent[b] = a;
+        return true;
+    }
+}
+```