Two stacks implement one queue, two queues implement one stack

Two stacks implement one queue:

The first stack is used for insertion, and the Second stack is used for deletion. The data in the two stacks is queue data, and the data flow is popped up from the first stack, and pushed into the Second stack without data. Therefore, the data capacity must be at least the length of one stack and at most two stacks.

Therefore, the first stack increases and the Second stack decreases.

Two queues implement one stack:

The first stack is used for insertion and deletion, and the Second stack is used to save advanced data. During insertion, data is always inserted from the first queue. If there is data in the first queue during deletion, the advanced data is put in the Second stack. After deletion, the data can be inserted again. Therefore, the data capacity can be at most the length of one queue.