树的直径

#include <iostream>
#include <vector>
#include <queue>
#include <string.h>
#include <algorithm>
using namespace std;

#define MAXN 100005

struct edge
{
    int from, to, weight;
};

vector<edge> vec[MAXN];
int n;

bool cmp(const int &a, const int &b)
{
    return a > b;
}

int d[MAXN];

void bfs(int r)
{
    queue<int> q;
    bool vis[MAXN] = {false};
    q.push(r);
    vis[r] = true;

    memset(d, 0, sizeof(d));
    while (!q.empty())
    {
        int u = q.front();
        q.pop();
        for (edge e : vec[u])
        {
            if (!vis[e.to])
            {
                d[e.to] = d[u] + e.weight;
                q.push(e.to);
                vis[e.to] = true;
            }
        }
    }
}

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);

    cin >> n;

    int s, t, w;
    edge tmp1, tmp2;
    for (int i = 0; i < n - 1; ++i)
    {
        cin >> s >> t >> w;
        tmp1.from = s;
        tmp1.to = t;
        tmp1.weight = tmp2.weight = w;
        tmp2.from = t;
        tmp2.to = s;
        vec[s].push_back(tmp1);
        vec[t].push_back(tmp2);
    }

    bfs(0);

    int m = -1, v;
    for (int i = 0; i < n; ++i)
        if (d[i] > m)
        {
            m = d[i];
            v = i;
        }

    bfs(v);

    sort(d, d + n, cmp);

    cout << d[0] << endl;
}