队列可以在队列的两端进行插入和删除操作,既可在队尾进行插入/删除,又可在队头进行插入/删除。现有4个不同的元素顺序输入到队列,那么可以得到_____种不同的排列。double-ended queue can insert and delete operations on both ends of the queue. That it can insert / delete at its tail, but also at the head. Existing 4 different elements sequentially input to the double-ended queue, you can get _____ different permutations.