2014 Super Training #9 E Destroy --树的直径+树形DP

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <cstdlib>
#include <algorithm>
#define ll long long
using namespace std;
#define N 10007

struct node
{
    int v,len,power;
    int next;
}G[4*N];

const ll Mod = (ll)(1LL<<60);
int head[N],tot,n;
ll dp[N];
int ma[N],sma[N];

void addedge(int u,int v,int len,int power)
{
    G[tot].v = v;
    G[tot].len = len;
    G[tot].power = power;
    G[tot].next = head[u];
    head[u] = tot++;
}

void dfs(int u,int fa)
{
    ma[u] = 0;
    sma[u] = 0;
    for(int i=head[u];i!=-1;i=G[i].next)
    {
        int v = G[i].v;
        if(v == fa)
            continue;
        dfs(v,u);
        int L = ma[v]+G[i].len;
        if(ma[u] < L)       // sma[u] < ma[u] < L
        {
            sma[u] = ma[u];
            ma[u] = L;
        }
        else if(sma[u] < L)   // sma[u] < L < ma[u]
            sma[u] = L;
    }
}

void DP(int u,int fa)
{
    for(int i=head[u];i!=-1;i=G[i].next)
    {
        int v = G[i].v;
        if(v == fa)
            continue;
        if(ma[u] == ma[v]+G[i].len)  //最远的点在v的子树内
        {
            ma[v] = max(ma[v],sma[u]+G[i].len);
            sma[v] = max(sma[v],sma[u]+G[i].len);
        }
        else
        {
            ma[v] = max(ma[v],ma[u]+G[i].len);
            sma[v] = max(sma[v],ma[u]+G[i].len);
        }
        DP(v,u);
    }
}

int findCenter()
{
    dfs(1,0);
    DP(1,0);
    int cen = min_element(ma+1,ma+n+1)-ma;
    return cen;
}

void dfs2(int u,int fa)
{
    int flag = 0;
    ll power = 0;
    dp[u] = Mod;
    for(int i=head[u];i!=-1;i=G[i].next)
    {
        int v = G[i].v;
        if(v == fa)
            continue;
        dfs2(v,u);
        power = max(power,min(dp[v],(ll)G[i].power));
        flag = 1;  //不是叶子节点
    }
    if(flag)
        dp[u] = power;
}

ll Get(int cen)
{
    dfs2(cen,0);
    return dp[cen];
}

int main()
{
    int u,v,len,power;
    int cen,i;
    while(scanf("%d",&n)!=EOF)
    {
        memset(head,-1,sizeof(head));
        tot = 0;
        for(i=0;i<n-1;i++)
        {
            scanf("%d%d%d%d",&u,&v,&len,&power);
            addedge(u,v,len,power);
            addedge(v,u,len,power);
        }
        cen = findCenter();
        printf("%lld\n",Get(cen));
    }
    return 0;
}