[BaekJoon] 11724번 - 연결 요소의 개수 [Java][C++]

문제 링크

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1. 문제 풀이

무방향 그래프에서 연결 요소의 개수를 구하는 문제로 BFS나 DFS를 활용해서 각 노드에 대해 방문하지 않았으면 모든 연결된 노드를 방문 처리하고 개수를 하나씩 세도 되며, 유니온 파인드 알고리즘을 활용해서 서로소 집합을 만든 후 집합의 그룹장의 개수를 세주어도 된다.

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2. 코드

1. BFS [Java]

import java.io.*;
import java.util.*;

public class Main {
    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        StringTokenizer st = new StringTokenizer(br.readLine());

        int N = Integer.parseInt(st.nextToken());
        int M = Integer.parseInt(st.nextToken());

        List<List<Integer>> adj = new ArrayList<>();
        for (int i = 0; i <= N; i++) {
            adj.add(new ArrayList<>());
        }

        for (int i = 0; i < M; i++) {
            st = new StringTokenizer(br.readLine());
            int u = Integer.parseInt(st.nextToken());
            int v = Integer.parseInt(st.nextToken());
            adj.get(u).add(v);
            adj.get(v).add(u);
        }

        boolean[] visited = new boolean[1 + N];
        int cnt = 0;
        for (int node = 1; node <= N; node++) {
            if (visited[node]) continue;

            bfs(node, visited, adj);
            cnt++;
        }

        System.out.println(cnt);
    }

    static void bfs(int start, boolean[] visited, List<List<Integer>> adj) {
        Queue<Integer> q = new ArrayDeque<>();
        q.offer(start);

        visited[start] = true;

        while (!q.isEmpty()) {
            int node = q.poll();

            for (int next : adj.get(node)) {
                if (visited[next]) continue;

                q.offer(next);
                visited[next] = true;
            }
        }
    }
}

2. DFS [Java]

import java.io.*;
import java.util.*;

public class Main {

    static List<List<Integer>> adj;
    static boolean[] visited;

    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        StringTokenizer st = new StringTokenizer(br.readLine());

        int N = Integer.parseInt(st.nextToken());
        int M = Integer.parseInt(st.nextToken());

        adj = new ArrayList<>();
        for (int i = 0; i <= N; i++) {
            adj.add(new ArrayList<>());
        }

        for (int i = 0; i < M; i++) {
            st = new StringTokenizer(br.readLine());
            int u = Integer.parseInt(st.nextToken());
            int v = Integer.parseInt(st.nextToken());
            adj.get(u).add(v);
            adj.get(v).add(u);
        }

        visited = new boolean[1 + N];
        int cnt = 0;
        for (int node = 1; node <= N; node++) {
            if (visited[node]) continue;

            dfs(node);
            cnt++;
        }

        System.out.println(cnt);
    }

    static void dfs(int node) {
        visited[node] = true;

        for (int next : adj.get(node)) {
            if (visited[next]) continue;

            dfs(next);
        }
    }
}

3. DSU [Java]

import java.io.*;
import java.util.*;

public class Main {

    static int[] p;

    static int[] make(int N) {
        int[] arr = new int[1 + N];
        for (int i = 1; i <= N; i++) {
            arr[i] = i;
        }
        return arr;
    }

    static int find(int x) {
        if (x == p[x]) return x;
        return p[x] = find(p[x]);
    }

    static void union(int x, int y) {
        p[find(y)] = find(x);
    }

    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        StringTokenizer st = new StringTokenizer(br.readLine());

        int N = Integer.parseInt(st.nextToken());
        int M = Integer.parseInt(st.nextToken());

        p = make(N);

        for (int i = 0; i < M; i++) {
            st = new StringTokenizer(br.readLine());
            int u = Integer.parseInt(st.nextToken());
            int v = Integer.parseInt(st.nextToken());
            union(u, v);
        }

        int cnt = 0;
        for (int node = 1; node <= N; node++) {
            if (node == find(node)) cnt++;
        }

        System.out.println(cnt);
    }
}

4. BFS [C++]

#include <bits/stdc++.h>
using namespace std;

int n, m;
vector<vector<int>> adj(1 + 1000);
bool visited[1 + 1000];

void bfs(int start) {
    queue<int> q;
    q.push(start);

    visited[start] = true;

    while (!q.empty()) {
        int node = q.front();
        q.pop();

        for (int next : adj[node]) {
            if (visited[next]) continue;

            q.push(next);
            visited[next] = true;
        }
    }
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    cin >> n >> m;

    for (int i = 0; i < m; i++) {
        int u, v;
        cin >> u >> v;
        adj[u].push_back(v);
        adj[v].push_back(u);
    }

    int cnt = 0;
    for (int node = 1; node <= n; node++) {
        if (visited[node]) continue;

        bfs(node);
        cnt++;
    }

    cout << cnt;
}

5. DFS [C++]

#include <bits/stdc++.h>
using namespace std;

int n, m;
vector<vector<int>> adj(1 + 1000);
bool visited[1 + 1000];

void dfs(int node) {
    visited[node] = true;

    for (int next : adj[node]) {
        if (visited[next]) continue;

        dfs(next);
    }
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    cin >> n >> m;

    for (int i = 0; i < m; i++) {
        int u, v;
        cin >> u >> v;
        adj[u].push_back(v);
        adj[v].push_back(u);
    }

    int cnt = 0;
    for (int node = 1; node <= n; node++) {
        if (visited[node]) continue;

        dfs(node);
        cnt++;
    }

    cout << cnt;
}

6. DSU [C++]

#include <bits/stdc++.h>
using namespace std;

int p[1 + 1000];

void make(int n) {
    for (int i = 1; i <= n; i++) p[i] = i;
}

int find(int x) {
    if (x == p[x]) return x;
    return p[x] = find(p[x]);
}

void unite(int x, int y) {
    p[find(y)] = find(x);
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int n, m;
    cin >> n >> m;

    make(n);

    for (int i = 0; i < m; i++) {
        int u, v;
        cin >> u >> v;
        unite(u, v);
    }

    int cnt = 0;
    for (int node = 1; node <= n; node++) {
        if (node == find(node)) cnt++;
    }

    cout << cnt;
}

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