Task for graph

#include <bits/stdc++.h>

using namespace std;

vector <set <int>> graph;


int main()
{
    int n;
    cin >> n;
    int a [n + 1];
    for (int i = 1; i <= n; i++)
        cin >> a [i];
    graph.resize(n + 1);
    for (int i = 1; i <= n; i++)
    {
        if (a [i] != -1)
            graph [a [i]].insert(i);
    }
    int ans = 0;
    int x = 0;
    bool visited [n + 1] = {};
    while (x != n)
    {
        for (int i = 1; i <= n; i++)
        {
            if (!visited [i] && graph [i].empty())
            {
                if (a [i] != -1)
                    graph [a [i]].erase(i);
                visited [i] = 1;
                x++;
            }
        }
        ans++;
    }
    cout << ans;
}