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;
}