BFS (with path print)

/*
BFS : O(E + V) .. shortetst path from source vertetx to another .. (unweighted graph ***)
*/
unordered_map<int,vector<int>> adj;  // adjacency list representation
int n; // number of nodes
int s; // source vertex given

queue<int> q;
vector<bool> used(n); // visited array
vector<int> d(n), p(n); // d is distance vertex and p is parent vertex ..

q.push(s);
used[s] = true;
p[s] = -1; // parent of source vertex is -1

while (!q.empty()) {
    int v = q.front();
    q.pop();
    for (int u : adj[v]) {
        if (!used[u]) {
            used[u] = true;
            q.push(u);
            d[u] = d[v] + 1;
            p[u] = v;
        }
    }
}

/// printing the shortets path from source to any vertex u ..
if (!used[u]) {
    cout << "No path!";
} else {
    vector<int> path;
    for (int v = u; v != -1; v = p[v])
        path.push_back(v);


    reverse(path.begin(), path.end());
    cout << "Path: ";
    for (int v : path)
        cout << v << " ";
}