判环

无向图

#include <bits/stdc++.h>
#include <vector>
#define PI acos(-1.0)
#define mem(a,b) memset((a),b,sizeof(a))
#define TS printf("!!!
")
#define pb push_back
//std::ios::sync_with_stdio(false);
using namespace std;
//priority_queue<int,vector<int>,greater<int>> que;
const double EPS = 1.0e-9;
typedef pair<int, int> pairint;
typedef long long ll;
typedef unsigned long long ull;
const int  maxn = 1e5 + 10;
const int  maxm = 300;
int visit[maxn];
int flag=0;
int dfs(int x, int pre)
{
        visit[x]=true;
        int len=p[x].size();
        for(int i=0;i<len;i++)
        {
                int to=p[x][i];
                if(to=pre)
                        continue;
                if(visit[to]==1)
                        flag=1,
　　　　　　　　　　　　    return 0;
else
                        dfs(to,x);
        }
}
int main()
{
        int n, m;
        cin  >> n >> m;
        for (int i = 1; i <= m; i++)
        {
                scanf("%d %d", &from, &to);
                p[from].pb(to);
                p[to].pb(from);
        }
        dfs(1,1);
        if(flag)
        cout<<"yes"<<endl;
        else
        cout<<"no"<<endl;
}

有向图

#include <bits/stdc++.h>
#include <vector>
#define PI acos(-1.0)
#define mem(a,b) memset((a),b,sizeof(a))
#define TS printf("!!!
")
#define pb push_back
//std::ios::sync_with_stdio(false);
using namespace std;
//priority_queue<int,vector<int>,greater<int>> que;
const double EPS = 1.0e-9;
typedef pair<int, int> pairint;
typedef long long ll;
typedef unsigned long long ull;
const int  maxn = 1e5 + 10;
const int  maxm = 300;
int visit[maxn];
int flag=0;
int dfs(int x, int pre)
{
        visit[x]=true;
        int len=p[x].size();
        for(int i=0;i<len;i++)
        {
                int to=p[x][i];
                if(visit[to]==1)
                        flag=1,
                        return ;
                else
                        dfs(to,x);
        }
}
int main()
{
        int n, m;
        cin  >> n >> m;
        for (int i = 1; i <= m; i++)
        {
                scanf("%d %d", &from, &to);
                p[from].pb(to);
        }
        dfs(1);
        if(flag)
        cout<<"yes"<<endl;
        else
        cout<<"no"<<endl;
}