强联通模板

#include<stdio.h>

#include<string.h>

#include<vector>

#include<stack>

using namespace std;

const int MN=2011;



vector<int>edge[MN];

vector<int>SCC[MN];

stack<int>s;



int low[MN],dfn[MN];

int instack[MN],stap[MN];

int belong[MN];

int num[MN];

int ind[MN];

int tp,p;

int bcnt;



void tarjan(int x)

{

    low[x]=dfn[x]=++tp;

    stap[++p]=x;

    instack[x]=1;

    for(int i=0;i<edge[x].size();i++)

    {

        int v=edge[x][i];

        if(dfn[v]==-1)

        {

            tarjan(v);

            if(low[x]>low[v])

               low[x]=low[v];

        }

        else if(instack[v] && dfn[v]<low[x])

               low[x]=dfn[v];

    }

    if(low[x]==dfn[x])

    {

        int top;

        do

        {

            top=stap[p];

            belong[top]=bcnt;//缩点

            instack[top]=0;

            p--;

        }while(x!=top);

        bcnt++;

    }

}



void topsort()//求入度

{

    memset(num,0,sizeof(num));

    int i,u,v;

    while(!s.empty())

        s.pop();

    for(int i=0;i<bcnt;i++)

        if(ind[i]==0)

        {

            s.push(i);

            num[i]++;

        }

    while(!s.empty())

    {

        u=s.top();

        s.pop();

        for(int i=0;i<SCC[u].size();i++)

        {

            v=SCC[u][i];

            if(--ind[v]==0)

            {

                s.push(v);

                num[v]=num[u]+1;

            }

        }

    }

}



int main()

{

    int T,n,m,i,a,b,j;

    scanf("%d",&T);

    while(T--)

    {

        bcnt=tp=p=0;

        scanf("%d%d",&n,&m);

        memset(dfn,-1,sizeof(dfn));

        memset(low,0,sizeof(low));

        memset(instack,0,sizeof(instack));

        memset(belong,0,sizeof(belong));

        memset(ind,0,sizeof(ind));

        for(i=1;i<=n;i++)

        {

            edge[i].clear();

            SCC[i].clear();

        }

        for(i=0;i<m;i++)

        {

            scanf("%d%d",&a,&b);

            edge[a].push_back(b);

        }

        for(i=1;i<=n;i++)

        {

            if(dfn[i]==-1) tarjan(i);

        }

        for(i=1;i<=n;i++)//缩点后建图

        {

            for(j=0;j<edge[i].size();j++)

            {

                int x=belong[i];

                int y=belong[edge[i][j]];

                if(x!=y)

                {

                     SCC[x].push_back(y);

                     ind[y]++;

                }

            }

        }

        topsort();

        int ans=-1;

        for(i=0;i<bcnt;i++)

        {

            ans=max(ans,num[i]);

        }

        if(bcnt==ans) printf("Yes\n");

        else printf("No\n");

    }

    return 0;

}