Print cycle in directed graph.cpp

/*
    CF 915D
    Print cycle in directed graph
*/

#include <bits/stdc++.h>

using namespace std;

#define MAX 505
int n, m;
vector<int> adj[MAX];
int cycle = -1;
int vis[MAX], par[MAX];
vector<int> V;
int x, y;

void dfs2(int u)
{
    vis[u] = 1;
    //cout << "u " << u << " " << vis[u] << endl;
    for(int i=0; i<adj[u].size(); i++){
        int v = adj[u][i];
        if(u == x && v == y) continue;
        //cout << "v " << v << " " << vis[v] << endl;
        if(vis[v] == 1){
            cycle = v;
            return;
        }
        if(vis[v] == 0){
            dfs2(v);
        }
    }
    vis[u] = 2;
}

void dfs(int u, int parent)
{
    vis[u] = 1;
    par[u] = parent;
    for(int i=0; i<adj[u].size(); i++){
        int v = adj[u][i];
        if((vis[v] == 1) && (cycle == -1)){
            cycle = v;
            V.push_back(cycle);
            int node = u;
            for(; node!= cycle; node=par[node]) V.push_back(node);
            V.push_back(cycle);
            reverse(V.begin(), V.end());
            return;
        }
        if(vis[v] == 0){
            dfs(v, u);
        }
    }
    vis[u] = 2;
}

int main()
{
    //freopen("in.txt", "r", stdin);
    scanf("%d %d", &n, &m);
    for(int i=0; i<m; i++){
        int u, v;
        scanf("%d %d", &u, &v);
        adj[u].push_back(v);
    }
    memset(vis, 0, sizeof vis);
    for(int i=1; i<=n; i++){
        if(vis[i] == 0){
            dfs(i, -1);
            if(cycle != -1) break;
        }
    }
    if(cycle == -1){
        cout << "YES";
        return 0;
    }
    memset(vis, 0, sizeof vis);
    for(int i=0; i<V.size()-1; i++){
        x = V[i], y = V[i+1], cycle = -1;
        memset(vis, 0, sizeof vis);
        for(int j=1; j<=n; j++){
            if(vis[j] == 0) dfs2(j);
        }
        if(cycle == -1){
            cout << "YES";
            return 0;
        }
    }
    cout << "NO";
}