Tree game

input/code.cpp: In function 'int main()':
input/code.cpp:32:5: error: 'stack' was not declared in this scope
   32 |     stack<int> st;
      |     ^~~~~
input/code.cpp:6:1: note: 'std::stack' is defined in header '<stack>'; did you forget to '#include <stack>'?
    5 | #include <queue>
  +++ |+#include <stack>
    6 | 
input/code.cpp:32:11: error: expected primary-expression before 'int'
   32 |     stack<int> st;
      |           ^~~
input/code.cpp:33:5: error: 'st' was not declared in this scope; did you mean 'std'?
   33 |     st.push(1);
      |     ^~
      |     std

#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <queue>

using namespace std;

using ll = long long;

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

    // freopen("input.txt", "r", stdin); // TODO: REMOVE THIS YOU STUPID ****

    int n;
    cin >> n;

    vector<ll> a(n+1);
    for (int i = 1; i <= n; ++i) cin >> a[i];

    vector<vector<int>> children(n+1);
    for (int i = 1; i <= n; ++i) {
        int p; cin >> p;
        if (i == 1) continue;
        children[p].push_back(i);
    }

    vector<int> order;
    order.reserve(n);
    stack<int> st;
    st.push(1);
    while (!st.empty()) {
        int v = st.top(); st.pop();
        order.push_back(v);
        for (int u : children[v]) st.push(u);
    }

    vector<ll> f(n+1, 0);
    for (int idx = (int)order.size() - 1; idx >= 0; --idx) {
        int v = order[idx];
        int k = (int)children[v].size();
        if (k <= 1) {
            f[v] = a[v];
        } else {
            ll max1 = 0, max2 = 0;
            for (int u : children[v]) {
                ll val = f[u];
                if (val > max1) {
                    max2 = max1;
                    max1 = val;
                } else if (val > max2) {
                    max2 = val;
                }
            }
            f[v] = a[v] + max2;
        }
    }

    cout << f[1];
    return 0;
}