方法一

#include <stdio.h>
#include <string.h>
#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <string>
#include <math.h>
#include <stdlib.h>
#include <time.h>
//#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N=1024;
struct Edge
{int s,t;
};
int n,m,dfn[N],low[N],in[N],id[N],cnt,dep,mp[N][N];
vector<Edge>edge;
vector<int>g[N];
stack<int>st;void Addedge(int u,int v)
{Edge tp;tp.s=u,tp.t=v;edge.push_back(tp);g[u].push_back(edge.size()-1);
}void dfs(int u)
{int v,i,k,mx=dfn[u]=low[u]=dep++;st.push(u);for(i=0;i<g[u].size();i++){v=edge[g[u][i]].t;if(dfn[v]==-1) dfs(v);if(mx>low[v]) mx=low[v];}if(mx<low[u]){low[u]=mx;return ;}//printf("%d\n",cnt);do{k=st.top();st.pop();id[k]=cnt;low[k]=n;}while(k!=u);cnt++;
}void dfs1(int u,int q)
{dfn[u]=1;dep=max(dep,q);for(int i=0;i<cnt;i++){if(mp[u][i]==1)dfs1(i,q+1);}
}
int main()
{//freopen("in.txt","r",stdin);//freopen("out.txt","w",stdout);int _,i,u,v;scanf("%d",&_);while(_--){scanf("%d%d",&n,&m);for(i=0; i<n; i++){g[i].clear();id[i]=i;}edge.clear();for(i=0; i<m; i++){scanf("%d%d",&u,&v);Addedge(u-1,v-1);}dep=0;cnt=0;while(!st.empty()) st.pop();memset(dfn,0xff,sizeof(dfn));for(i=0; i<n; i++){if(dfn[i]==-1)dfs(i);}memset(in,0,sizeof(in));memset(mp,0,sizeof(mp));for(i=0; i<m; i++){u=id[edge[i].s];v=id[edge[i].t];//printf("%d %d\n",u,v);if(u!=v){in[v]++;mp[u][v]=1;}}memset(dfn,0,sizeof(dfn));for(i=0,dep=0;i<cnt;i++){if(dfn[i]==0) dfs1(i,1);}if(dep==cnt) printf("Yes\n");else printf("No\n");}return 0;
}

方法二

#include <stdio.h>
#include <string.h>
#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <string>
#include <math.h>
#include <stdlib.h>
#include <time.h>
//#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N=1024;
struct Edge
{int s,t;
};
int n,m,dfn[N],low[N],in[N],id[N],cnt,dep,mp[N][N];
vector<Edge>edge;
vector<int>g[N];
stack<int>st;void Addedge(int u,int v)
{Edge tp;tp.s=u,tp.t=v;edge.push_back(tp);g[u].push_back(edge.size()-1);
}void dfs(int u)
{int v,i,k,mx=dfn[u]=low[u]=dep++;st.push(u);for(i=0;i<g[u].size();i++){v=edge[g[u][i]].t;if(dfn[v]==-1) dfs(v);if(mx>low[v]) mx=low[v];}if(mx<low[u]){low[u]=mx;return ;}//printf("%d\n",cnt);do{k=st.top();st.pop();id[k]=cnt;low[k]=n;}while(k!=u);cnt++;
}int topsort()
{int i,u,v,p;stack<int>S;for(i=0;i<cnt;i++){//printf("%d\n",in[i]);if(in[i]==0)S.push(i);}//printf("%d\n",S.size());p=0;while(S.size()==1){u=S.top();S.pop();p++;for(i=0;i<cnt;i++){if(mp[u][i]==1){in[i]--;if(in[i]==0) S.push(i);}}}if(p==cnt) return 1;else return 0;
}int main()
{//freopen("in.txt","r",stdin);//freopen("out.txt","w",stdout);int _,i,u,v;scanf("%d",&_);while(_--){scanf("%d%d",&n,&m);for(i=0; i<n; i++){g[i].clear();id[i]=i;}edge.clear();for(i=0; i<m; i++){scanf("%d%d",&u,&v);Addedge(u-1,v-1);}dep=0;cnt=0;while(!st.empty()) st.pop();memset(dfn,0xff,sizeof(dfn));for(i=0; i<n; i++){if(dfn[i]==-1)dfs(i);}memset(in,0,sizeof(in));memset(mp,0,sizeof(mp));for(i=0; i<m; i++){u=id[edge[i].s];v=id[edge[i].t];if(u!=v&&!mp[u][v]){in[v]++;mp[u][v]=1;}}if(topsort()) printf("Yes\n");else printf("No\n");}return 0;
}

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