【模板】Tarjan缩点

洛谷3387

 #include<cstdio>
 #include<algorithm>
 #include<cstring>
 using namespace std;
 const int maxn=,maxm=;
 int ans,n,m,etot,tot,top,color,last[maxn],w[maxn],f[maxn],val[maxn],dfn[maxn],low[maxn],col[maxn],st[maxn];
 struct edge{int pre,to;}e[maxm];
 struct rec{int x,y;}a[maxm];
 void read(int &k){
     k=; int f=; char c=getchar();
     while(c<''||c>'')c=='-'&&(f=-),c=getchar();
     while(''<=c&&c<='')k=k*+c-'',c=getchar();
     k*=f;
 }
 int max(int a,int b){if (a>b) return a; return b;}
 int min(int a,int b){if (a<b) return a; return b;}
 void add(int x,int y){e[++etot]=(edge){last[x],y}; last[x]=etot;}
 void tarjan(int x){
     dfn[x]=low[x]=++tot; st[++top]=x;
     for (int i=last[x],to;i;i=e[i].pre)
         if (!dfn[to=e[i].to]) tarjan(to),low[x]=min(low[x],low[to]);
         else if (!col[to]) low[x]=min(low[x],dfn[to]);
     if (dfn[x]==low[x])
         for(color++;st[top+]!=x;top--) col[st[top]]=color,val[color]+=w[st[top]];
 }
 void dfs(int x){
     if (f[x]) return; f[x]=val[x]; int tmp=;
     for (int i=last[x],to;i;i=e[i].pre) dfs(to=e[i].to),tmp=max(tmp,f[to]);
     f[x]+=tmp;
 }
 int main(){
     read(n); read(m);
     for (int i=;i<=n;i++) read(w[i]);
     for (int i=;i<=m;i++) read(a[i].x),read(a[i].y),add(a[i].x,a[i].y);
     for (int i=;i<=n;i++) if (!dfn[i]) tarjan(i);
     etot=; memset(last,,sizeof(last));
     for (int i=,x,y;i<=m;i++) if(col[x=a[i].x]!=col[y=a[i].y]) add(col[x],col[y]);
     for (int i=;i<=color;i++) dfs(i),ans=max(ans,f[i]);
     return printf("%d\n",ans),;
 }