HDUOJ:2196

樹的直徑（樹形dp）：

兩次dfs，一次由下至上，一次又上至下。

#include <bits/stdc++.h>
using namespace std;
#define maxn 10005

int n;
vector<pair<int,int> > m[maxn];
int deep1[maxn];
int deep1id[maxn];
int deep2[maxn];
int ans[maxn];

void init(){
    memset(deep1,0,sizeof(deep1));
    memset(deep1id,0,sizeof(deep1id));
    memset(deep2,0,sizeof(deep2));
    memset(ans,0,sizeof(ans));
    for(int i=0;i<maxn;i++) vector<pair<int,int> >().swap(m[i]);
}

int getdis(int a,int fa){
    for(int i=0;i<m[a].size();i++){
        if(m[a][i].first == fa) continue;
        int c = m[a][i].first;
        int d = m[a][i].second;
        int t = getdis(c,a) + d;
        if(t >= deep2[a]) deep2[a] = t;
        if(deep2[a] >= deep1[a]) swap(deep1[a],deep2[a]),deep1id[a] = c;
    }
    return deep1[a];
}

void putdis(int a,int fa,int key){
    ans[a] = max(deep1[a],key);
    for(int i=0;i<m[a].size();i++){
        if(m[a][i].first == fa) continue;
        int c = m[a][i].first;
        int d = m[a][i].second;
        if(c == deep1id[a]) putdis(c,a,max(deep2[a],key)+d);
        else putdis(c,a,max(deep1[a],key)+d);
    }
}

void print(){ for(int i=1;i<=n;i++) printf("%d\n",ans[i]);}

int main(){
    while(scanf("%d",&n)!=EOF){
        init();
        for(int i=2;i<=n;i++){
            int a,b;
            scanf("%d%d",&a,&b);
            m[i].push_back(make_pair(a,b));
            m[a].push_back(make_pair(i,b));
        }
        m[0].push_back(make_pair(1,0));
        m[1].push_back(make_pair(0,0));
        getdis(1,0);
        putdis(1,0,0);
        print();
    }
    return 0;
}