C++ function to find tree diameter

tree_diameter.cpp

#include <bits/stdc++.h>

using namespace std;
typedef long long ll;

template <typename T>
class general_tree {
   private:
    T root;
    bool modified = false;
    unordered_map<T, vector<T>> graph;
    unordered_map<T, T> parent;
    unordered_map<T, ll> subtree_size, depth;
    void pre_calc_size(ll cur_node) {
        subtree_size[cur_node] = 1;
        for (T i : graph[cur_node]) {
            if (i == parent[cur_node]) continue;
            depth[cur_node]++;
            pre_calc_size(i);
            subtree_size[cur_node] += subtree_size[i];
        }
    }
    void farthest_node(T &ans_node, ll &max_distance, T cur_node, T prev_node = -1, ll distance = 0) {
        for (T i : graph[cur_node]) {
            if (i == prev_node) continue;
            farthest_node(ans_node, max_distance, i, cur_node, distance + 1);
        }
        if (distance > max_distance) {
            max_distance = distance;
            ans_node = cur_node;
        }
    }

   public:
    general_tree(T tree_root) {
        root = tree_root;
    }
    void connect(T parent_node, T children_node) {
        parent[children_node] = parent_node;
        graph[parent_node].push_back(children_node);
        graph[children_node].push_back(parent_node);
        modified = true;
    }
    ll size(T node) {
        if (modified) {
            pre_calc_size(root);
            modified = false;
        }
        return subtree_size[node];
    }
    ll diameter() {
        T a;
        ll x = -1;
        farthest_node(a, x, root);
        x = -1;
        T b;
        farthest_node(b, x, a);
        return x;
    }
};

int main() {
    ll n;
    cin >> n;
    general_tree<ll> tree(1);
    for (ll i = 2; i <= n; i++) {
        ll x;
        cin >> x;
        tree.connect(x, i);
    }
    cout << tree.diameter();
}