GRANDFATHER PARADOX

#include <bits/stdc++.h>
#define nax 100003

using namespace std;

vector<int> a[nax];
int n, s, rad;

void Citire()
{
    int x;
    cin >> n;
    for(int i = 1; i <= n; ++i)
    {
        cin >> x;
        if(!x)
            rad = i;
        else
            a[x].push_back(i);
    }
}

void DFS(int node)
{
    ++s;
    for(int& it : a[node])
        DFS(it);
}

int main()
{
    Citire();
    DFS(rad);
    cout << (s == n ? "DA" : "NU");
    return 0;
}