[haoi2009]毛毛虫 树形dp

 1 #include<iostream>
 2 #include<cstdio>
 3 #include<cstring>
 4 using namespace std;
 5 const int maxn=303000;
 6 int n,m,f[maxn],child[maxn];
 7 struct node{
 8     int y,next;
 9 }e[maxn<<1];
10 int linkk[maxn],len=0;
11 void insert(int x,int y){
12     e[++len].y=y;
13     e[len].next=linkk[x];
14     linkk[x]=len;
15 }
16 void init(){
17     scanf("%d%d",&n,&m);
18     int x,y;
19     for(int i=1;i<=m;i++){
20         scanf("%d%d",&x,&y);
21         insert(x,y);insert(y,x);
22     }
23 }
24 void dfs(int x,int fa){
25     for(int i=linkk[x];i;i=e[i].next){
26         if(e[i].y==fa)continue;
27         child[x]++;
28         dfs(e[i].y,x);
29     }
30 }
31 int ans=0;
32 void Dfs(int x,int fa){
33     if(child[x]==0){
34         f[x]=1;return;
35     }
36     int maxx[2]={0,0};
37     for(int i=linkk[x];i;i=e[i].next){
38         if(e[i].y==fa)continue;
39         Dfs(e[i].y,x);
40         f[x]=max(f[x],f[e[i].y]+child[x]);
41         if(f[e[i].y]>maxx[0])maxx[0]=f[e[i].y];
42         if(f[e[i].y]>maxx[1])maxx[0]=maxx[1],maxx[1]=f[e[i].y];
43     }
44     ans=max(ans,f[x]+maxx[0]-1);
45     if(x!=1)ans=max(ans,f[x]+maxx[0]);
46     if(maxx[0]==0)ans=max(ans,f[x]);
47 }
48 void work(){
49     dfs(1,0);
50     Dfs(1,0);
51     printf("%d\n",ans);
52 }
53 int main(){
54     freopen("1.in","r",stdin);
55     freopen("1.out","w",stdout);
56     init();
57     work();
58 }