学步园

#include<iostream>
#include<vector>
#include <cstring>
#include <set>
#include<cstdio>
using namespace std;

const int MAX=100011;

int Stop;//栈中的元素个数
int cnt;//记录连通分量的个数
int visitNum;//记录遍历的步数
int DFN[MAX]; //记录节点u第一次被访问时的步数
int LOW[MAX]; //记录与节点u和u的子树节点中最早的步数
bool instack[MAX];//记录节点u是否在栈中
int Stap[MAX];//栈
int Belong[MAX];//记录每个节点属于的强连通分量编号

int N;//节点个数

vector<int> tree[MAX],scc[MAX];

void tarjan(int i)
{
    int j;
    DFN[i]=LOW[i]=++visitNum;
    instack[i]=true;
    Stap[++Stop]=i;//将当前节点压入栈中
    for (unsigned k=0;k<tree[i].size();k++)
    {
        j=tree[i][k];
        if (!DFN[j])
        {
            tarjan(j);
            if (LOW[j]<LOW[i])
                LOW[i]=LOW[j];
        }
        else if (instack[j] && DFN[j]<LOW[i])
            LOW[i]=DFN[j];
    }
    if (DFN[i]==LOW[i])
    {
        cnt++;
      //  cout<<"连通分量"<<cnt<<": ";
        scc[cnt].clear();
        do
        {
            j=Stap[Stop--];
            instack[j]=false;
          //  cout<<j<<" ";
            scc[cnt].push_back(j);
            Belong[j]=cnt;
        }
        while (j!=i);
            //    cout<<endl;
    }
}
int deg[MAX];
vector<int> son[MAX];
set<int> VIS;
void solve()
{
    Stop=cnt=visitNum=0;
    memset(DFN,0,sizeof(DFN));
    for (int i=1;i<=N;i++)
        if (!DFN[i])//有可能图不是连通图
            tarjan(i);

    for(int i=1;i<=cnt;i++){
          son[i].clear();
          VIS.clear();
          deg[i] = scc[i].size();
       //   cout<<i<<" "<<deg[i]<<": ";
          for(int j=0;j<scc[i].size();j++){
              int u = scc[i][j];
              for(int k=0;k<tree[u].size();k++){
                  if(Belong[tree[u][k]]!=i && !VIS.count(Belong[tree[u][k]])){
                      VIS.insert(Belong[tree[u][k]]);
                      son[i].push_back(Belong[tree[u][k]]);
                     // cout<<Belong[tree[u][k]]<<" ";
                  }
              }
          }
        //  cout<<endl;
    }
}
int c[MAX],t,topo[MAX];
bool Dfs(int u){
c[u] = -1;
for(int i=0;i<son[u].size();i++){
     int v = son[u][i];
     if(c[v] < 0) return false;
     else if(!c[v] && !Dfs(v)) return false;
}
c[u] = 1; topo[--t]=u;
return true;
}
bool toposort(){
t = cnt;
memset(c,0,sizeof(c));
for(int u = 1;u<=cnt;u++) if(!c[u]){
      if(!Dfs(u)) return false;
}
return true;
}
int d[MAX],vv[MAX];
int dp(int u,int fa){
if(vv[u]) return d[u];
vv[u]=1;
int res = deg[u];
for(int i=0;i<son[u].size();i++)if(son[u][i]!=fa){
    res = max(res,dp(son[u][i],u)+deg[u]);
}
return d[u] = res;
}
int main()
{
    int m;
    while(scanf("%d %d",&N,&m)==2){
          for(int i=1;i<=N;i++) tree[i].clear();
          for(int i=1;i<=m;i++){
              int x,y;
              scanf("%d %d",&x,&y);
              tree[x].push_back(y);
          }
          solve();
          toposort();
          int res = 0;
          memset(vv,0,sizeof(vv));
          for(int i=0;i<cnt;i++){
            int u = topo[i];
            if(!vv[u]){
                 res = max(res,dp(u,-1));
                 //if(res == 7) cout<<u<<"**\n";
            }
          }
          printf("%d\n",res);
    }
    return 0;
}
/*
6 8
1 2
1 3
3 4
4 1
2 4
4 6
3 5
5 6

6 */