알고리즘 풀이/BFS(너비 우선 탐색)

BOJ 2146 [다리 만들기]

갓우태 2018. 10. 4. 06:28

BOJ 다리 만들기 : https://www.acmicpc.net/problem/2146





안녕하세요. 이번 문제는 백준 온라인 저지의 [다리 만들기] 입니다.



1. BFS로 현재 위치에서 갈 수 있는 곳을 확장시킨 뒤, BFS를 빠져나올 때 마다 해당 섬에 번호 부여.


2. 다시 처음부터 BFS를 돌며, 첫 시작지와 섬 번호가 다른 섬을 찾아 떠난다.







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import java.util.*;
class Node {
    public int ori_num, i, j, depth;
    public Node(int ori_num, int i, int j, int depth) {
        this.ori_num = ori_num;
        this.i = i;
        this.j = j;
        this.depth = depth;
    }
}
public class Main {
    public static int N;
    public static int[][] Map = new int[100][100];
    public static boolean[][] Visited = new boolean[100][100];
    public static int[][] d = {{-1, 0}, {1, 0}, {0, -1}, {0, 1}};
    public static int island_num = 1;            //각각의 섬의 번호 
    public static int Result = 99999999;
    public static Queue<Node> q = new LinkedList<Node>();
    public static Queue<Node> q2 = new LinkedList<Node>();
    public static boolean is_bound(int i, int j) {
        return i >= 0 && i < N && j >= 0 && j < N;
    }
    public static void BFS(int select) {
        while(!q.isEmpty()) {
            Node node = q.poll();
            q2.offer(node);
            if(select == 1) {
                if(Map[node.i][node.j] != node.ori_num  && Map[node.i][node.j] != 0) {
                    Result = Result < node.depth -1 ? Result : node.depth -1;
                }
            }
            for(int k = 0; k < 4; k++) {
                int next_i = node.i + d[k][0];
                int next_j = node.j + d[k][1];
                if(is_bound(next_i, next_j) && !Visited[next_i][next_j]) {
                    if(select == 0 && Map[next_i][next_j] != 0) {
                        Visited[next_i][next_j] = true;
                        Map[next_i][next_j] = island_num;
                        q.offer(new Node(0, next_i, next_j, 0));
                    }
                    else if(select == 1 && Map[next_i][next_j] != node.ori_num) {
                        Visited[next_i][next_j] = true;
                        q.offer(new Node(node.ori_num, next_i, next_j, node.depth+1));
                    }
                }
            }
        }
    }
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        N = sc.nextInt();
        for(int i = 0; i < N; i++)
            for(int j = 0; j < N; j++)
                Map[i][j] = sc.nextInt();
        for(int i = 0; i < N; i++) {
            for(int j = 0; j < N; j++) {
                if(!Visited[i][j] && Map[i][j] != 0) {
                    Visited[i][j] = true;
                    Map[i][j] = island_num;    //현재 탐색하는 섬에 번호부여 
                    q.offer(new Node(0, i, j, 0));
                    BFS(0);
                    island_num++;
                }
            }
        }
        while(!q2.isEmpty()) {
            Node node = q2.poll();
            Visited[node.i][node.j] = false; 
        }
        for(int i = 0; i < N; i++) {
            for(int j = 0; j < N; j++) {
                if(Map[i][j] != 0) {
                    Visited[i][j] = true;
                    q.offer(new Node(Map[i][j], i, j, 0));
                    BFS(1);
                }
                while(!q2.isEmpty()) {
                    Node node = q2.poll();
                    Visited[node.i][node.j]= false; 
                }
            }
        }
        System.out.println(Result);
    }
}
cs


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