hdu 4514 树形DP求树直径

#include <string.h>
#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <string>
#include <math.h>
#define maxn 1000001
using namespace std;
typedef pair<int, int> pii;
typedef pair<long long, long long> pll;
typedef vector<int> vi;
typedef vector<long long> vll;
inline int read(){int ra,fh;char rx;rx=getchar(),ra=0,fh=1;while((rx<'0'||rx>'9')&&rx!='-')rx=getchar();if(rx=='-')fh=-1,rx=getchar();while(rx>='0'&&rx<='9')ra*=10,ra+=rx-48,rx=getchar();return ra*fh;}
//#pragma comment(linker, "/STACK:102400000,102400000")
 
struct Edge
{
    int to,nx,w;
}edge[maxn*2];
 
int head[maxn],fa[maxn],dp[maxn],ans;
int cnt,n,m;
void addedge(int u,int v,int w)
{
    edge[cnt] = Edge {v,head[u],w};
    head[u] = cnt++;
    edge[cnt] = Edge {u,head[v],w};
    head[v] = cnt++;
}
int find(int x){return f[x]==x?x:f[x]=find(f[x]);}
 
void dfs(int u,int father)
{
    int mx = 0;
    for(int i=head[u];i;i=edge[i].nx)
    {
        int v = edge[i].to;
        int w = edge[i].w;
        if(v==father)continue;
        dfs(v,u);
        ans = max(ans,dp[v]+dp[u]+w);
        dp[u] = max(dp[u],dp[v]+w);
    }
}
 
void solve()
{
    for(int i=1;i<=n;i++)  
        if(dp[i]==-1)
            dfs(i,-1);
    
    printf("%d\n",ans);
}
 
int main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    while(~scanf("%d%d",&n,&m))
    {
        int flag = 0;
        ans = 0,cnt = 1;
        for(int i=1;i<=n;i++)
        {
            fa[i] = i;
            dp[i] = -1;
            head[i] = 0;
        }
        for(int i=1;i<=m;i++)
        {
            int u,v,w;
            scanf("%d%d%d",&u,&v,&w);
            addedge(u,v,w);
            if(flag)continue;
            int fx,fy;
            fx = Find(u), fy = Find(v);
            if(fx!=fy) fa[fx] = fy; 
            else flag = 1;
        }
        if(flag)
            printf("YES\n"),continue;   
        solve();
    }
    return 0;
}

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