BFS Pseudocode

  /*
    * queue.push(start)
    * step = 0
    * while (queue.isNotEmpty()){
    *   step++
    *   size = queue.size
    *   //scan current level
    *   while (size-->0){
    *       node = queue.pop()
    *       //从node开始扩展
    *       new_nodes = expand(node)
    *       if (goal in new_nodes){
    *           return step++
    *       }
    *       queue.append(new_nodes)
    *   }
    * }
    * return NOT_FOUND
    * */