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0225.implement-stack-using-queues

题目

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:

  1. 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.
  2. 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.
  3. You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

Credits:Special thanks to @jianchao.li.fighter for adding this problem and all test cases.

解题思路

见程序注释