一道题两种解法：深度优先搜索、广度优先搜索-AtCode ABC204 C - Tour

N M
A1 B1
...
AM BM

3 3
1 2
2 3
3 2

7

3 0

3

4 4
1 2
2 3
3 4
4 1

16

int n;
vector<bool>leaf(n);

void dfs(int x, vector<vector<int>> &vvi) {
    if(leaf[x]) {
        return;
    }
    leaf[x] = true;
    for(auto a : vvi[x]) {
        dfs(a, vvi);
    }
}

void coder_solution() {
    // 提升cin、cout效率
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    int m, temp = 0;
    cin >> n >> m;
    vector<vector<int>> vvi(n);
    int a, b;
    for (int i = 0; i < m; i++) {
        cin >> a >> b;
        vvi[a - 1].push_back(b - 1);
    }
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            leaf[j] = false;
        }
        dfs(i, vvi);
        for (int j = 0; j < n; j++) {
            if (leaf[j]) {
                temp++;
            }
        }
    }
}

void best_solution() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    int n, m;
    cin >> n >> m;
    int a, b;
    vector<vector<int>> vvi(n);
    for(int i = 0; i < m; i++) {
        cin >> a >> b;
        vvi[a - 1].push_back(b - 1);
    }

    vector<int> dist(n, -1);
    queue<int> que;
    dist[0] = 0;
    int v;
    int j = 0;
    for(int i = 0; i < n; i++) {
        que.push(i);
        dist[i] = 0;
        while(!que.empty()) {
            v = que.front();
            que.pop();
            for(auto next_v : vvi[v]) {
                if (dist[next_v] != -1) {
                    continue;
                }
                dist[next_v] = dist[v] + 1;
                que.push(next_v);
            }
        }
        j += n - count(dist.begin(), dist.end(), -1);
        dist.assign(n, -1);
    }
    cout << j;
}

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

int main(){
    int n, m;
    cin >> n >> m;
    auto e = vector(n, vector<int>());
    for(int i{}, a, b; i < (m); ++i){
        cin >> a >> b;
        --a, --b;
        e[a].push_back(b);
    }
    ll ans{};
    for(int s{}; s< (n); ++s){
        vector<bool> vis(n);
        vis[s] = true;
        queue<int> q;
        q.push(s);
        while(!q.empty()){
            ++ans;
            int i = q.front();
            q.pop();
            for(auto&j:e[i]) if(!vis[j]){
                vis[j] = true;
                q.push(j);
            }
        }
    }
    cout << ans << endl;
}

#include<bits/stdc++.h>

using namespace std;
const int MAX_N = 2000;
vector<vector<int>> G;
bool temp[MAX_N];

void dfs(int v) {
    if (temp[v]) {
        return;
    }
    temp[v] = true;
    for (auto vv: G[v]){
        dfs(vv);
    }
}

int main() {
    int N, M;
    cin >> N >> M;
    G.resize(N);
    for (int i = 0; i < M; i++) {
        int a, b;
        cin >> a >> b;
        G[a - 1].push_back(b - 1);
    }
    int ans = 0;
    for (int i = 0; i < N; i++) {
        for (int j = 0; j < N; j++) {
            temp[j] = false;
        }
        dfs(i);
        for (int j = 0; j < N; j++) {
            if (temp[j]){
                ans++;
            |
        }
    }
    cout << ans;
}

dist.assign(n, -1);