HDU2196 Computer 树形dp

分析：两遍dfs的树形dp裸题，记录子树最长链，然后自顶向下更新即可。。。

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <string>
#include <cmath>
#include <stdlib.h>
using namespace std;
typedef long long LL;
const int N=1e4+5;
int T,n,head[N],tot,id[N];
LL ret[N],mx[N];
struct Edge{
  int v,next;
  LL w;
}edge[N<<1];
void add(int u,int v,LL w){
  edge[tot].v=v;
  edge[tot].w=w;
  edge[tot].next=head[u];
  head[u]=tot++;
}
void dfs(int u,int f){
  mx[u]=0;id[u]=-1;
  for(int i=head[u];~i;i=edge[i].next){
     int v=edge[i].v;if(v==f)continue;dfs(v,u);
     if(mx[v]+edge[i].w>mx[u]){mx[u]=mx[v]+1ll*edge[i].w;id[u]=v;}
  }
}
void get(int u,int f,LL x){
  LL p=mx[u];ret[u]=max(mx[u],x);
  if(x>=p){p=x;id[u]=f;}
  for(int i=head[u];~i;i=edge[i].next){
    int v=edge[i].v;if(v==f)continue;
    if(v!=id[u]){get(v,u,p+edge[i].w);continue;}
    LL tmp=x;
    for(int j=head[u];~j;j=edge[j].next){
      int to=edge[j].v;if(to==f||to==v)continue;
      tmp=max(tmp,mx[to]+edge[j].w);
    }
    get(v,u,tmp+edge[i].w);
  }
}
int main(){
  while(~scanf("%d",&n)){
   for(int i=1;i<=n;++i)head[i]=-1;
    tot=0;
   for(int i=2;i<=n;++i){
     int u,w;scanf("%d%d",&u,&w);
     add(u,i,w);add(i,u,w);
   }
   dfs(1,-1);get(1,-1,0);
   for(int i=1;i<=n;++i)printf("%I64d\n",ret[i]);
  }
  return 0;
}