Leetcode: Day 112, ##, Count Complete Tree Nodes, Rectangle Area, Basic Calculator, Implement Stack using Queues, Invert Binary Tree

Leetcode: Day 112, ##, Count Complete Tree Nodes, Rectangle Area, Basic Calculator, Implement Stack using Queues, Invert Binary Tree

Implement Stack using Queues

Implement the following operations of a stack using queues.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• empty() -- Return whether the stack is empty.

Notes:

• You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, and is empty operations are valid.
• Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
• You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

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用queue的大小来控制倒腾，只用一个queue，而不是两个

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