hdu 2196 树形dp

思路：先求以1为根时，每个节点到子节点的最大长度。然后再次从1进入进行更新。

#include<iostream>
#include<cstring>
#include<cstdio>
#include<algorithm>
#include<vector>
#define Maxn 20010
#define inf 0x7fffffff
using namespace std;
int vi[Maxn],si[Maxn],n,road[Maxn],Max[Maxn],lMax[Maxn],head[Maxn],e;
struct Edge{
    int u,v,val,next;
}edge[Maxn*2];
void init()
{
    memset(vi,0,sizeof(vi));
    memset(si,0,sizeof(si));
    memset(road,0,sizeof(road));
    memset(Max,0,sizeof(Max));
    memset(lMax,0,sizeof(lMax));
    memset(head,-1,sizeof(head));
    e=0;
}
void add(int u,int v,int val)
{
    edge[e].u=u,edge[e].v=v,edge[e].val=val,edge[e].next=head[u],head[u]=e++;
    edge[e].u=v,edge[e].v=u,edge[e].val=val,edge[e].next=head[v],head[v]=e++;
}
int dfs(int u)
{
    int i,v,sz;
    vi[u]=1;
    for(i=head[u];i!=-1;i=edge[i].next)
    {
        v=edge[i].v;
        if(vi[v]) continue;
        int temp=dfs(v);
        if(temp+edge[i].val>Max[u])
        {
            lMax[u]=Max[u];
            Max[u]=temp+edge[i].val;
            road[u]=v;
        }
        else if(temp+edge[i].val>lMax[u])
            lMax[u]=temp+edge[i].val;
    }
    return Max[u];
}
void update(int u,int val)
{
    int i,v,sz;
    vi[u]=1;
    for(i=head[u];i!=-1;i=edge[i].next)
    {
        v=edge[i].v;
        if(vi[v]) continue;
        if(road[u]==v) update(v,max(val,lMax[u])+edge[i].val);
        else update(v,max(Max[u],val)+edge[i].val);
    }
    Max[u]=max(Max[u],val);
}
int main()
{
    int i,j,a,b;
    while(scanf("%d",&n)!=EOF)
    {
        init();
        for(i=2;i<=n;i++){
            scanf("%d%d",&a,&b);
            add(i,a,b);
        }
        dfs(1);
        memset(vi,0,sizeof(vi));
        update(1,0);
        for(i=1;i<=n;i++)
            printf("%d\n",Max[i]);
    }
    return 0;
}