BZOJ 4890: [Tjoi2017]城市 树形dp

code: 

#include <bits/stdc++.h> 
using namespace std;   
#define setIO(s) freopen(s".in","r",stdin) 
const int N=5010; 
int n,cnt=1,ans=453533453,tmp;   
int hd[N],to[N<<1],nex[N<<1],dis[N<<1],f[N],g[N],maxson[N],check[N<<1],val[N<<1];     
inline void add(int u,int v,int c) 
{
    nex[++cnt]=hd[u],hd[u]=cnt,to[cnt]=v,val[cnt]=c;  
}   
void dfs1(int u,int ff) 
{
    for(int i=hd[u];i;i=nex[i]) 
    {
        int v=to[i];  
        if(v==ff||check[i])   continue;   
        dfs1(v,u);   
        tmp=max(tmp,f[u]+f[v]+val[i]);          
        if(f[v]+val[i]>f[u])   g[u]=f[u],f[u]=f[v]+val[i],maxson[u]=v;   
        else if(f[v]+val[i]>g[u])  g[u]=f[v]+val[i];  

    }
} 
void dfs2(int x,int ff,int maxx) 
{
    tmp=min(tmp,max(maxx,f[x]));   
    for(int i=hd[x];i;i=nex[i]) 
    {
        int v=to[i];  
        if(v==ff||check[i])   continue;     
        if(maxson[x]==v)  dfs2(v,x,max(g[x]+val[i],maxx+val[i]));    
        else dfs2(v,x,max(f[x]+val[i],maxx+val[i]));    
    }
}
inline void init() 
{
    memset(f,0,sizeof(f)); 
    memset(g,0,sizeof(g));  
    memset(maxson,0,sizeof(maxson));  
}
int main() 
{  
    // setIO("input"); 
    int i,j;    
    scanf("%d",&n); 
    for(i=1;i<n;++i) 
    {
        int u,v,w; 
        scanf("%d%d%d",&u,&v,&w);   
        add(u,v,w),add(v,u,w);                 
    }  
    for(i=2;i<=cnt;i+=2) 
    {
        init();   
        int d1,d2,d3,d4,x=to[i],y=to[i^1];  
        check[i]=check[i^1]=1;  
        tmp=0, dfs1(x,0), d1=tmp;  
        tmp=0, dfs1(y,0), d2=tmp;   
        tmp=10000000, dfs2(x,0,0), d3=tmp;              
        tmp=10000000, dfs2(y,0,0), d4=tmp;      
        ans=min(ans,max(max(d1,d2),d3+d4+val[i]));  
        check[i]=check[i^1]=0; 
    } 
    printf("%d\n",ans); 
    return 0; 
}