BZOJ 3887: [Usaco2015 Jan]Grass Cownoisseur tarjan + spfa

Code: 

#include <bits/stdc++.h>
#define setIO(s) freopen(s".in","r",stdin) 
#define maxn 200002
using namespace std;     
queue<int>Q;  
stack<int>S; 
vector<int>G[maxn],V[maxn]; 
map<int,int>mp[maxn];   
int n,m,edges,scc,id; 
int hd[maxn],to[maxn],nex[maxn],low[maxn],dfn[maxn],idx[maxn],vis[maxn],siz[maxn];     
void addedge(int u,int v) {
    nex[++edges]=hd[u],hd[u]=edges,to[edges]=v; 
} 
void tarjan(int u) {
    S.push(u); 
    vis[u]=1, low[u]=dfn[u]=++scc; 
    for(int i=hd[u];i;i=nex[i]) {
        int v=to[i]; 
        if(!vis[v]) tarjan(v), low[u]=min(low[u], low[v]); 
        else if(vis[v]==1) low[u]=min(low[u], dfn[v]); 
    }
    if(low[u]==dfn[u]) {
        ++id; 
        for(;;) { 
            int x=S.top(); S.pop(); 
            idx[x]=id;   
            ++siz[id];     
            vis[x]=-1;  
            if(x==u) break;   
        }
    }
} 
int h1[maxn],h2[maxn],  vis1[maxn],vis2[maxn],inq[maxn], ge[maxn];     
void spfa1() { 
    int s=idx[1];  
    Q.push(s);   
    inq[s]=1;  
    h1[s]=siz[s];   
    while(!Q.empty()) {
        int u=Q.front(); Q.pop();  
        ge[u]=1;  
        inq[u]=0; 
        for(int i=0;i<G[u].size();++i) {        
            int v=G[u][i]; 
            if(h1[v] < h1[u] + siz[v]) {
                h1[v] = h1[u] + siz[v]; 
                if(!inq[v]) {
                    Q.push(v); 
                    inq[v]=1;   
                }
            }
        }
    }                                     
}      
void spfa2() { 
    int s=idx[1];  
    Q.push(s);   
    inq[s]=1;  
    h2[s]=siz[s];        
    while(!Q.empty()) {
        int u=Q.front(); Q.pop(); 
        inq[u]=0; 
        for(int i=0;i<V[u].size();++i) {
            int v=V[u][i]; 
            if(h2[v] < h2[u] + siz[v]) {
                h2[v] = h2[u] + siz[v]; 
                if(!inq[v]) {
                    Q.push(v); 
                    inq[v]=1;   
                }
            }
        }
    }
}       
int main() {
    // setIO("input"); 
    scanf("%d%d",&n,&m); 
    for(int i=1;i<=m;++i) {
        int x,y;        
        scanf("%d%d",&x,&y); 
        addedge(x,y);   
    }   
    for(int i=1;i<=n;++i) if(!vis[i]) tarjan(i);  
    for(int i=1;i<=n;++i) {     
        int cur=idx[i]; 
        for(int j=hd[i];j;j=nex[j]) {
            int too=idx[to[j]]; 
            if(cur!=too && !mp[cur][too]) {
                mp[cur][too]=1; 
                G[cur].push_back(too);   
                V[too].push_back(cur);  
            }
        }
    }        
    spfa2();               
    spfa1(); 
    int ans=siz[idx[1]]; 
    int re=0;  
    for(int i=1;i<=id;++i) { 
        if(!ge[i]) continue; 
        for(int j=0;j<V[i].size();++j) 
        {
            int v=V[i][j];                                
            if(h1[i] && h2[v])ans=max(ans, h1[i] + h2[v]-siz[idx[1]]);           
        }
    }
    printf("%d\n",ans);        
    return 0; 
}