缩点×2

感觉已经掌握了tarjan的真正思想。。

#include <cstdio>
using namespace std;

const int maxn=1e4+5, maxm=1e5+5;

inline int max(int a, int b) { return a<b?b:a; }
inline int min(int a, int b) { return a<b?a:b; }

class Graph{
public:
    struct Edge{
        Graph *belong;
        int to, next;
        Edge operator ++(){ *this=belong->edge[next]; return *this; }
        int operator *(){ return to; }
    };
    void addedge(int a, int b){
        Edge &e=edge[++cntedge];
        e.to=b, e.next=fir[a];
        fir[a]=cntedge, e.belong=this;
    }
    Edge gete(int x) { return edge[fir[x]]; }
    Edge edge[maxm];
private:
    int cntedge, fir[maxn];
};

int n, m, tail, time, cntscc, ans;
int v[maxn], dfn[maxn], low[maxn];
int stack[maxn], instack[maxn];
int belong[maxn], in[maxn], v_scc[maxn], dp[maxn];
Graph g, g_scc;

void tarjan(int now){
    dfn[now]=low[now]=++time;
    stack[++tail]=now, instack[now]=1;
    int nowson;
    for (Graph::Edge e=g.gete(now); *e; ++e){
        nowson=*e;
        //写在前面比较好，尽管无伤大雅
        if (instack[nowson])
            low[now]=min(low[now], dfn[nowson]);
        if (!dfn[nowson]){
            tarjan(nowson);
            low[now]=min(low[now], low[nowson]);
        }
    }
    if (dfn[now]==low[now]){
        ++cntscc;
        for (; low[stack[tail]]!=dfn[stack[tail]]; --tail){
            instack[stack[tail]]=0; //这里忘记了！！
            belong[stack[tail]]=cntscc;
            v_scc[cntscc]+=v[stack[tail]];
        }
        instack[stack[tail]]=0;
        belong[stack[tail]]=cntscc;
        v_scc[cntscc]+=v[stack[tail]];
        --tail;
    }
}

void dfs(int now, int par){
    int nowson;
    for (Graph::Edge e=g_scc.gete(now); *e; ++e){
        nowson=*e;
        if (nowson==par) continue;
        if (!dp[nowson]) dfs(nowson, now);
        dp[now]=max(dp[now], dp[nowson]);
    }
    dp[now]+=v_scc[now];
    ans=max(ans, dp[now]);
}

int main(){
    scanf("%d%d", &n, &m);
    for (int i=1; i<=n; ++i) scanf("%d", &v[i]);
    int x, y;
    for (int i=0; i<m; ++i){ scanf("%d%d", &x, &y); g.addedge(x, y); }
    for (int i=1; i<=n; ++i)
        if (!dfn[i]) tarjan(i);
    for (int i=1; i<=n; ++i)
        for (Graph::Edge e=g.gete(i); *e; ++e){
            if (belong[i]!=belong[*e]){
                g_scc.addedge(belong[i], belong[*e]);
                ++in[belong[*e]];
            }
        }
    for (int i=1; i<=cntscc; ++i)
        if (!in[i]) dfs(i, 0);
    printf("%d
", ans);
}