先随机求离一个位置最远的点
再求离这个点最远的点即可

# include <bits/stdc++.h>

using namespace std;

const int N = 1e5 + 10;
long long d[N], n, x, y, z, h, t, q[N], dmax, Mi;
bool st[N];
vector<int> s[N], sd[N];

int main()
{
    cin >> n;
    for(int i = 1; i < n; i ++)
    {
        cin >> x >> y >> z;
        s[x].push_back(y);
        s[y].push_back(x);
        sd[x].push_back(z);
        sd[y].push_back(z);
    }

    h = t = 0;
    q[0] = 1;
    d[1] = 0;
    memset(st, false, sizeof st);
    st[1] = true;
    while(h <= t)
    {
        int out = q[h ++];

        for(int i = 0; i < s[out].size(); i ++)
        {
            int ne = s[out][i];
            if(!st[ne])
            {
                st[ne] = true;
                d[ne] = d[out] + sd[out][i];
                q[++t] = ne;
            }
        }
    }

    dmax = 0;
    for(int i = 1; i <= n; i ++)
        if(dmax < d[i]) dmax = d[i], Mi = i;

    h = t = 0;
    q[0] = Mi;
    d[Mi] = 0;
    memset(st, false, sizeof st);
    st[Mi] = true;
    while(h <= t)
    {
        int out = q[h ++];

        for(int i = 0; i < s[out].size(); i ++)
        {
            int ne = s[out][i];
            if(!st[ne])
            {
                st[ne] = true;
                d[ne] = d[out] + sd[out][i];
                q[++t] = ne;
            }
        }
    }

    dmax = 0;
    for(int i = 1; i <= n; i ++) dmax = max(dmax, d[i]);

    dmax = dmax * 10 + (1 + dmax) * dmax / 2;
    cout << dmax;
}