缩点×2

 1 #include <cstdio>
 2 using namespace std;
 3 
 4 const int maxn=1e4+5, maxm=1e5+5;
 5 
 6 inline int max(int a, int b) { return a<b?b:a; }
 7 inline int min(int a, int b) { return a<b?a:b; }
 8 
 9 class Graph{
10 public:
11     struct Edge{
12         Graph *belong;
13         int to, next;
14         Edge operator ++(){ *this=belong->edge[next]; return *this; }
15         int operator *(){ return to; }
16     };
17     void addedge(int a, int b){
18         Edge &e=edge[++cntedge];
19         e.to=b, e.next=fir[a];
20         fir[a]=cntedge, e.belong=this;
21     }
22     Edge gete(int x) { return edge[fir[x]]; }
23     Edge edge[maxm];
24 private:
25     int cntedge, fir[maxn];
26 };
27 
28 int n, m, tail, time, cntscc, ans;
29 int v[maxn], dfn[maxn], low[maxn];
30 int stack[maxn], instack[maxn];
31 int belong[maxn], in[maxn], v_scc[maxn], dp[maxn];
32 Graph g, g_scc;
33 
34 void tarjan(int now){
35     dfn[now]=low[now]=++time;
36     stack[++tail]=now, instack[now]=1;
37     int nowson;
38     for (Graph::Edge e=g.gete(now); *e; ++e){
39         nowson=*e;
40         //写在前面比较好，尽管无伤大雅
41         if (instack[nowson])
42             low[now]=min(low[now], dfn[nowson]);
43         if (!dfn[nowson]){
44             tarjan(nowson);
45             low[now]=min(low[now], low[nowson]);
46         }
47     }
48     if (dfn[now]==low[now]){
49         ++cntscc;
50         for (; low[stack[tail]]!=dfn[stack[tail]]; --tail){
51             instack[stack[tail]]=0; //这里忘记了！！
52             belong[stack[tail]]=cntscc;
53             v_scc[cntscc]+=v[stack[tail]];
54         }
55         instack[stack[tail]]=0;
56         belong[stack[tail]]=cntscc;
57         v_scc[cntscc]+=v[stack[tail]];
58         --tail;
59     }
60 }
61 
62 void dfs(int now, int par){
63     int nowson;
64     for (Graph::Edge e=g_scc.gete(now); *e; ++e){
65         nowson=*e;
66         if (nowson==par) continue;
67         if (!dp[nowson]) dfs(nowson, now);
68         dp[now]=max(dp[now], dp[nowson]);
69     }
70     dp[now]+=v_scc[now];
71     ans=max(ans, dp[now]);
72 }
73 
74 int main(){
75     scanf("%d%d", &n, &m);
76     for (int i=1; i<=n; ++i) scanf("%d", &v[i]);
77     int x, y;
78     for (int i=0; i<m; ++i){ scanf("%d%d", &x, &y); g.addedge(x, y); }
79     for (int i=1; i<=n; ++i)
80         if (!dfn[i]) tarjan(i);
81     for (int i=1; i<=n; ++i)
82         for (Graph::Edge e=g.gete(i); *e; ++e){
83             if (belong[i]!=belong[*e]){
84                 g_scc.addedge(belong[i], belong[*e]);
85                 ++in[belong[*e]];
86             }
87         }
88     for (int i=1; i<=cntscc; ++i)
89         if (!in[i]) dfs(i, 0);
90     printf("%d\n", ans);
91 }