【模板】Tarjan缩点

洛谷3387

#include<cstdio>
#include<algorithm>
#include<cstring>
using namespace std;
const int maxn=20010,maxm=200010;
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=0; int f=1; char c=getchar();
    while(c<'0'||c>'9')c=='-'&&(f=-1),c=getchar();
    while('0'<=c&&c<='9')k=k*10+c-'0',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+1]!=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=0;
    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=1;i<=n;i++) read(w[i]);
    for (int i=1;i<=m;i++) read(a[i].x),read(a[i].y),add(a[i].x,a[i].y);
    for (int i=1;i<=n;i++) if (!dfn[i]) tarjan(i);
    etot=0; memset(last,0,sizeof(last));
    for (int i=1,x,y;i<=m;i++) if(col[x=a[i].x]!=col[y=a[i].y]) add(col[x],col[y]);
    for (int i=1;i<=color;i++) dfs(i),ans=max(ans,f[i]);
    return printf("%d
",ans),0;
}