1 //TshingHua OJ 旅行商(TSP)
 2 //邻接表+DP+队列+拓扑排序
 3 //Memory:66304KB Time:1101Ms(No.17)
 4 #include<iostream>
 5 #include<cstring>
 6 #include<cstdio>
 7 using namespace std;
 8 
 9 #define MAX 1000005
10 #define Max(x,y) ((x)>(y)?(x):(y))
11 
12 int n, m;    //村庄个数-道路数
13 int topology[MAX],lt;    //拓扑数组-长度
14 int mark[MAX];    //入度标记
15 int maxCity = 1;    //答案
16 
17 //从City通向的村庄
18 struct Node{
19     int num;    //村庄编号
20     Node *next;
21     Node(){ next = NULL; }
22     Node(int x,Node *n) :num(x),next(n){}
23 };
24 
25 struct City{
26     Node *nc;    //next-city
27     int dp;        //至此可通过的最大城市数
28     City(){ nc = NULL; dp = 1; }
29     void insert(int nc);
30 }city[MAX];
31 
32 void City::insert(int nc)
33 {
34     mark[nc]++;    //直接后继城市入度+1
35     if (this->nc == NULL)
36         this->nc = new Node(nc,NULL);
37     else{
38         Node *node = new Node(nc,this->nc);
39         this->nc = node;
40     }
41     return;
42 }
43 
44 /*拓扑排序*/
45 void Topology()
46 {
47     for (int i = 1; i <= n; i++)
48         if (!mark[i]) topology[++lt] = i;    //入度为0的city
49     //Main Content
50     for (int i = 1; topology[i];i++)
51     {
52         int cur = topology[i];    //该city-number
53         //遍历该city所有直接后继
54         for (Node *tmp = city[cur].nc; tmp != NULL; tmp = tmp->next)
55         {
56             //此处满足无后效性-DP
57             city[tmp->num].dp = Max(city[cur].dp + 1, city[tmp->num].dp);
58             maxCity = Max(city[tmp->num].dp, maxCity);
59             //处理后继
60             int num = tmp->num;
61             mark[num]--;    //后继入度-1
62             if (!mark[num])    topology[++lt] = num;    //若后继入度为0
63         }
64     }
65 }
66 
67 int main()
68 {
69     scanf("%d%d", &n, &m);
70     for (int i = 0; i < m; i++)
71     {
72         int x, y;    //x->y
73         scanf("%d%d", &x, &y);
74         city[x].insert(y);
75     }
76     /*拓扑排序*/
77     Topology();
78     printf("%d\n", maxCity);
79 
80     return 0;
81 }