leetcode207. 课程表——图—环检测

#include 
#define endl '\n'
#define int long long
using namespace std;
constexpr int N = 1e5, M = N, inf = 0x3f3f3f3f;
vector<int> g[N];
bool onpath[N]; // 记录一次递归堆栈中的节点
bool vis[N];    // 记录遍历过的节点，防止走回头路
bool cycle;     // 记录图中是否有环
void dfs(int u)
{
    if (onpath[u])
    {
        cycle = true;
    }
    if (vis[u] || cycle)
    {
        return;
    }
    // 标记节点u已经遍历，且开始遍历节点u；
    vis[u] = onpath[u] = true;
    for (const auto &it : g[u])
    {
        dfs(it);
    }
    onpath[u] = false; // 标记尾部取消；
}
void solve()
{
    int n, m;
    cin >> m;
    for (int i = 0; i < m; i++)
    {
        int a, b;
        cin >> a >> b;
        g[a].push_back(b);
    }
    for (int i = 0; i < m; i++)
    {
        dfs(i);
    }
    if (cycle)
    {
        cout << "yes" << endl;
    }
    else
        cout << "no" << endl;
}
signed main()
{
    ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
 
    solve();
 
    return 0;
}