如题。

#include <bits/stdc++.h>
using namespace std;
#define fast_io ios::sync_with_stdio(0), cin.tie(nullptr)
#define endl '\n'
#define int long long
#define ar array<int, 2>
#define arr array<int, 3>
int T, n, m, k, inf = 1e18, mod = 1e9 + 7;
const int N = 3e5 + 50;
vector<ar> mp[N];

ar dfs(int u, int p)
{
    int mx = 0;
    int l = 0;
    for (auto [v, t] : mp[u])
    {
        if (v == p)
            continue;
        auto [x, y] = dfs(v, u);
        mx = max(x + t - y, mx);
        l += y;
    }
    return {l, max(l, l + mx)};
};
signed main()
{
    fast_io;
#ifdef DEBUG
    freopen("../1.in", "r", stdin);
#endif

    cin >> n;
    for (int i = 1; i < n; ++i)
    {
        int x, y, z;
        cin >> x >> y >> z;
        mp[x].push_back({y, z});
        mp[y].push_back({x, z});
    }
    // 为什么从一个入度为0 的边开始走 反而不行？？？
    // for (int i = 1; i <= n; ++i)
    //     if (mp[i].size() == 1)
    //     {
    //         dfs(i, 0);
    //     }
    auto a = dfs(1, 0);
    cout << max(a[0], a[1]);
};
//场景:
//量级:
//搞: