/**
 * C++
 * Return the length of the shortest path between root and target node.
 */
int BFS(Node root, Node target) {
    queue<Node> queue;  // store all nodes which are waiting to be processed
    unordered_set<Node> visited;  // store all the nodes that we've visited
    step = 0;       // number of steps neeeded from root to current node

    // initialize
    add root to queue;
    add root to visited;

    // BFS
    while (queue is not empty) {
        step = step + 1;

        // iterate the nodes which are already in the queue
        int size = queue.size();
        for (int i = 0; i < size; ++i) {
            Node cur = queue.front(); //the first node in queue
            queue.pop();// remove the first node from queue
            return step if cur is target;

            for (Node next : the neighbors of cur) {
                if (visited.count(next) == 0 /*next is not in visited*/) {
                    queue.push(next); //add next to queue
                    visited.insert(next); //add next to visited
                }  
            }
        }
    }

    return -1;  // there is no path from root to target
}