求树的直径

#include <iostream>
#include <algorithm>
#include <string>
#include <string.h>
#include <vector>
#include <map>
#include <stack>
#include <set>
#include <queue>
#include <math.h>
#include <cstdio>
#include <iomanip>
#include <time.h>
#include <bitset>
#include <cmath>

#define LL long long
#define INF 0x3f3f3f3f
#define ls nod<<1
#define rs (nod<<1)+1

const double eps = 1e-10;
const int maxn = 1e6 + 10;
const LL mod = 1e9 + 7;

int sgn(double a){return a < -eps ? -1 : a < eps ? 0 : 1;}
using namespace std;

struct edge {
    int v,w,nxt;
}e[maxn];

int head[maxn],f1[maxn],f2[maxn];
int cnt,ans;
void add_edge(int u,int v,int w) {
    e[++cnt].nxt = head[u];
    e[cnt].w = w;
    e[cnt].v = v;
    head[u] = cnt;
}


void dfs(int u,int f) {
    f1[u] = f2[u] = 0;
    for (int i = head[u];~i;i = e[i].nxt) {
        int v = e[i].v;
        if (v == f)
            continue;
        dfs(v,u);
        if (f1[u] < f1[v] + e[i].w) {
            f2[u] = f1[u];
            f1[u] = f1[v] + e[i].w;
        }
        else if (f2[u] < f1[v] + e[i].w)
            f2[u] = f1[v] + e[i].w;
    }
    ans = max(ans,f1[u]+f2[u]);
}

int main() {
    //freopen("../in.txt","r",stdin);
    memset(f1,0,sizeof(f1));
    memset(f2,0, sizeof(f2));
    memset(head,-1, sizeof(head));
    cnt = 0,ans = 0;
    int u,v,w;
    while (~scanf("%d%d%d",&u,&v,&w)) {
        add_edge(u,v,w);
        add_edge(v,u,w);
    }
    dfs(1,0);
    printf("%d\n",ans);
    return 0;
}