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*];
void init()
{
memset(vi,,sizeof(vi));
memset(si,,sizeof(si));
memset(road,,sizeof(road));
memset(Max,,sizeof(Max));
memset(lMax,,sizeof(lMax));
memset(head,-,sizeof(head));
e=;
}
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]=;
for(i=head[u];i!=-;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]=;
for(i=head[u];i!=-;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=;i<=n;i++){
scanf("%d%d",&a,&b);
add(i,a,b);
}
dfs();
memset(vi,,sizeof(vi));
update(,);
for(i=;i<=n;i++)
printf("%d\n",Max[i]);
}
return ;
}