Finding Center of TREE

#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define io ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0)
const int MAX = 1e6 + 5;
vector <int> a[MAX];
queue <int> q;
int used[MAX], n, ln[MAX], lm[MAX], x;

void zero(int n){
    for(int i = 1; i <= n; i++){
        ln[i] = 0;
        used[i] = 0;
    }
}

void BFS(int node){
    q.push(node);
    used[node] = 1;
    while(!q.empty()){
        int s = q.front();
        q.pop();
        for(int i : a[s]){
            if(!used[i]){
                q.push(i);
                used[i] = 1;
                ln[i] = ln[s] + 1;
            }
        }
    }
}

int main(){
    io;
    cin >> n;
    if(n == 1){
        cout << 1;
        return 0;
    }
    for(int i = 2; i <= n; i++){
        cin >> x;
        a[x].push_back(i);
        a[i].push_back(x);
    }
    BFS(1);
    int d = 0, d1, d2;
    for(int i = 1; i <= n; i++){
        if(ln[i] > d){
            d = ln[i];
            d1 = i;
        }
    }
    d = 0;
    zero(n);
    BFS(d1);
    for(int i = 1; i <= n; i++){
        lm[i] = ln[i];
        if(ln[i] > d){
            d = ln[i];
            d2 = i;
        }
    }
    zero(n);
    BFS(d2);
    int r1 = d / 2;
    int r2 = d / 2;
    if(d % 2 == 1){
        r2++;
    }
    for(int i = 1; i <= n; i++){
        if(ln[i] == r1 && lm[i] == r2){
            cout << i << " ";
        }
        else if(ln[i] == r2 && lm[i] == r1){
            cout << i << " ";
        }
    }
}