# P3119 [USACO15JAN]Grass Cownoisseur G [ Tarjan + 缩点 + 拓扑序 + dp + 最长路] [好题]

## 题目描述

In an effort to better manage the grazing patterns of his cows, Farmer John has installed one-way cow paths all over his farm. The farm consists of N fields, conveniently numbered 1..N, with each one-way cow path connecting a pair of fields. For example, if a path connects from field X to field Y, then cows are allowed to travel from X to Y but not from Y to X.

Bessie the cow, as we all know, enjoys eating grass from as many fields as possible. She always starts in field 1 at the beginning of the day and visits a sequence of fields, returning to field 1 at the end of the day. She tries to maximize the number of distinct fields along her route, since she gets to eat the grass in each one (if she visits a field multiple times, she only eats the grass there once).

As one might imagine, Bessie is not particularly happy about the one-way restriction on FJ's paths, since this will likely reduce the number of distinct fields she can possibly visit along her daily route. She wonders how much grass she will be able to eat if she breaks the rules and follows up to one path in the wrong direction. Please compute the maximum number of distinct fields she can visit along a route starting and ending at field 1, where she can follow up to one path along the route in the wrong direction. Bessie can only travel backwards at most once in her journey. In particular, she cannot even take the same path backwards twice.

## 输入格式

INPUT: (file grass.in)

The first line of input contains N and M, giving the number of fields and the number of one-way paths (1 <= N, M <= 100,000).

The following M lines each describe a one-way cow path. Each line contains two distinct field numbers X and Y, corresponding to a cow path from X to Y. The same cow path will never appear more than once.

## 输出格式

OUTPUT: (file grass.out)

A single line indicating the maximum number of distinct fields Bessie

can visit along a route starting and ending at field 1, given that she can

follow at most one path along this route in the wrong direction.

```7 10
1 2
3 1
2 5
2 4
3 7
3 5
3 6
6 5
7 2
4 7

```

```6
```

## 说明/提示

SOLUTION NOTES:

Here is an ASCII drawing of the sample input:

``````v---3-->6
7   |  |
^  v  |
|  1   |
|   |   v
|   v   5
4<--2---^
``````

Bessie can visit pastures 1, 2, 4, 7, 2, 5, 3, 1 by traveling

backwards on the path between 5 and 3. When she arrives at 3 she

cannot reach 6 without following another backwards path. 相当坎坷的一道紫题,数据也不算水了, 前前后后大概想了两个小时, 写了一个小时, wa了一个小时.

首先, 知道这题是求最长路;

其次, 注意到反向只能走一次这个问题, 怎么做到只能反向走一次.

先把点分成三种情况来讨论一下,

1 直接可以由 1 号草坪走来的;

2 可以走到 1 号草坪的;

3 和 1 号点完全没有任何关系的.

显然这道题根本不会用到第 3 这种情况, 所以只需要建立一个正图和一个反向图, 在DAG上跑最长路, 显然可以由 1 号节点做起点在拓扑序上跑dp就可以了.

最后枚举每条反边, ans 就是 反向走到此条反边的正向图的父亲边的距离 + 正向走到此条反边的距离 - 1 号节点所在的连通块大小( 就是这里wa穿了, 因为有数据是1号节点直接在一个连通块里的 ), 并维护 ans 的最大值即可.

CODE  ```  1 #include <bits/stdc++.h>
2 #define dbg(x) cout << #x << "=" << x << endl
3
4 using namespace std;
5 typedef long long LL;
6 const int maxn = 1e5 + 7;
7
8 int head[maxn], dfn[maxn], low[maxn], st[maxn];
9 int cnt = 0, tot = 0, tim = 0, top = 1, n, m, cl = 0, ans = 0;
10 int vis[maxn];
11 int color[maxn];
12 int sz[maxn];
13 int dis[maxn];
14 int head1[maxn << 1], cnt1, edge1[maxn << 1], nxt1[maxn << 1];
15 int in[maxn << 1];
16
17 /*
19
20 dfn[],low[]:tarjan中数组
21
22 st[]:模拟栈
23
24 out[]:出边
25
26 sd[]:强连通分量存储
27
28 dq[]:统计答案
29 */
30
31 template<class T>inline void read(T &res)
32 {
33     char c;T flag=1;
34     while((c=getchar())<'0'||c>'9')if(c=='-')flag=-1;res=c-'0';
35     while((c=getchar())>='0'&&c<='9')res=res*10+c-'0';res*=flag;
36 }
37
38 struct Edge{
39     int nxt, to;
40 }edge[maxn * 2];
41
42 inline void BuildGraph(int from, int to)
43 {
44     cnt++;
45     edge[cnt].to = to;
48 }
49
50 void tarjan(int x)
51 {
52     tim++;
53     dfn[x] = low[x] = tim;
54     st[top] = x;
55     top++;
56     vis[x] = 1;
57     for(int i = head[x] ; i != 0; i = edge[i].nxt)
58     {
59         int u = edge[i].to;
60         if(vis[u] == 0)
61         {
62             tarjan(u);
63             low[x]=min(low[x],low[u]);
64         }
65         else if(vis[u] == 1)
66                 low[x]=min(low[x],dfn[u]);
67     }
68     if(dfn[x] == low[x])
69     {
70         cl++;
71         do
72         {
73             top--;
74             color[st[top]] = cl;
75             vis[st[top]] = -1;
76             sz[color[st[top]]]++;
77         }while( st[top] != x );
78     }
79     return ;
80 }
81
82 void addedge(int u, int v, int cas) {
83     if(cas == 1) {
84         cnt++;
85     }
86     in[v][cas]++;
87     edge1[cnt][cas] = v;
90 }
91
92 void topo(int cas) {
93     dis[color][cas] = sz[color];
94     queue<int> q;
95     for ( int i = 1; i <= cl; ++i ) {
96         if(in[i][cas] == 0) {
97             q.push(i);
98         }
99     }
100     while(!q.empty()) {
101         int u = q.front();
102         q.pop();
103         for ( int i = head1[u][cas]; i; i = nxt1[i][cas] ) {
104             int v = edge1[i][cas];
105             dis[v][cas] = max(dis[v][cas], dis[u][cas] + sz[v]);
106             if(--in[v][cas] == 0) {
107                 q.push(v);
108             }
109         }
110     }
111 }
112
113 int main()
114 {
115     scanf("%d %d",&n, &m);
116     for ( int i = 1; i <= m; ++i ) {
117        int x, y;
118        scanf("%d %d",&x, &y);
119        BuildGraph(x, y);
120     }
121     for ( int i = 1; i <= n; ++i ) {
122         if( !vis[i] ) {
123             tarjan(i);
124         }
125     }
126     cnt = 0;
127     for ( int i = 1; i <= n; ++i ) {
128         for ( int j = head[i]; j; j = edge[j].nxt ) {
129             int v = edge[j].to;
130             if(color[i] != color[v]) {
133             }
134         }
135     }
136     memset(dis, 0xef, sizeof(dis));
137     ans = sz[color];
138     topo(1), topo(2);
139     for ( int i = 1; i <= n; ++i ) {
140         for ( int j = head[i]; j; j = edge[j].nxt ) {
141             int v = edge[j].to;
142             if(color[i] != color[v]) {
143                 ans = max(ans, dis[color[v]] + dis[color[i]] - sz[color]);
144             }
145         }
146     }
147     cout << ans << endl;
148     return 0;
149 }```
View Code

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