check if bfs is possible or not

// C++ implementation to check if the
// given permutation is a valid
// BFS of a given tree
 
#include <bits/stdc++.h>
using namespace std;
 
// map for storing the tree
map<int, vector<int> > tree;
 
// map for marking
// the nodes visited
map<int, int> vis;
 
// Function to check if
// permutation is valid
bool valid_bfs(vector<int>& v)
{
    int n = (int)v.size();
    queue<set<int> > q;
    set<int> s;
    s.insert(1);
 
    /*inserting the root in 
     the front of queue.*/
    q.push(s);
    int i = 0;
 
    while (!q.empty() && i < n) {
 
        // If the current node
        // in a permutation
        // is already visited
        // then return false
        if (vis.count(v[i])) {
            return 0;
        }
        vis[v[i]] = 1;
 
        // if all the children of previous
        // nodes are visited then pop the
        // front element of queue.
        if (q.front().size() == 0) {
            q.pop();
        }
 
        // if the current element of the
        // permutation is not found
        // in the set at the top of queue
        // then return false
        if (q.front().find(v[i])
            == q.front().end()) {
            return 0;
        }
        s.clear();
 
        // push all the children of current
        // node in a set and then push
        // the set in the queue.
        for (auto j : tree[v[i]]) {
            if (vis.count(j)) {
                continue;
            }
            s.insert(j);
        }
        if (s.size() > 0) {
            set<int> temp = s;
            q.push(temp);
        }
        s.clear();
 
        // erase the current node from
        // the set at the top of queue
        q.front().erase(v[i]);
 
        // increment the index
        // of permutation
        i++;
    }
 
    return 1;
}
 
// Driver code
int main()
{
    tree[1].push_back(2);
    tree[2].push_back(1);
    tree[1].push_back(5);
    tree[5].push_back(1);
    tree[2].push_back(3);
    tree[3].push_back(2);
    tree[2].push_back(4);
    tree[4].push_back(2);
    tree[5].push_back(6);
    tree[6].push_back(5);
 
    vector<int> arr
        = { 1, 5, 2, 3, 4, 6 };
 
    if (valid_bfs(arr))
        cout << "Yes" << endl;
 
    else
        cout << "No" << endl;
 
    return 0;
}