int main(){
    d(V, INF);
    d[s] = 0; // distance from source s to s is 0
    queue<int> q;
    q.push(s); // start from source
    while (!q.empty()) {
        int u = q.front();
        q.pop(); // queue: layer by layer!
        for (int j = 0; j < (int)AdjList[u].size(); j++) {
            ii v = AdjList[u][j]; // for each neighbor of u
            if (d[v.first] == INF) {
                // if v.first is unvisited + reachable
                d[v.first] = d[u] + 1; // make d[v.first] != INF to flag it
                q.push(v.first); // enqueue v.first for the next iteration
            }
        }
    }
}