dfs

#include <iostream>
#include <cstring>

using namespace std;

const int N = 110, M = 2010;

int n, m;
int h[N], e[M], ne[M], idx;
bool st[N];

void add(int a, int b)
{
    e[idx] = b, ne[idx] = h[a], h[a] = idx++;
}

void dfs(int u)
{
    st[u] = true;
    for (int i = h[u]; i != -1; i = ne[i])
    {
        int j = e[i];
        if (!st[j]) dfs(j);
    }
}

int main()
{
    cin >> n >> m;
    memset(h, -1, sizeof h);
    while (m--)
    {
        int a, b;
        cin >> a >> b;
        add(a, b);
    }

    dfs(1);

    for (int i = 1; i <= n; i++)
    {
        if (!st[i])
        {
            printf("-1\n");
            return 0;
        }
    }
    printf("1\n");
}

bfs

#include <iostream>
#include <cstring>
#include <queue>

using namespace std;

const int N = 110, M = 2010;

int n, m;
int h[N], e[M], ne[M], idx;
bool st[N];

void add(int a, int b)
{
    e[idx] = b, ne[idx] = h[a], h[a] = idx++;
}

void bfs()
{
    queue<int> q;
    q.push(1);
    st[1] = true;

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

        for (int i = h[t]; i != -1; i = ne[i])
        {
            int j = e[i];
            if (!st[j])
            {
                q.push(j);
                st[j] = true;
            }
        }
    }
}

int main()
{
    cin >> n >> m;
    memset(h, -1, sizeof h);
    while (m--)
    {
        int a, b;
        scanf("%d%d", &a, &b);
        add(a, b);
    }
    bfs();
    for (int i = 1; i <= n; i++) // 遍历结点1~n
    {
        if (!st[i])
        {
            cout << -1 << endl;
            return 0;
        }
    }
    cout << 1 << endl;
}