[LeetCode] 232. Implement Queue using Stacks 用栈来实现队列

拼搏现实的明天。 2022-10-02 12:52 107阅读 0赞

Implement the following operations of a queue using stacks.

*  push(x) -- Push element x to the back of queue.
 *  pop() -- Removes the element from in front of queue.
 *  peek() -- Get the front element.
 *  empty() -- Return whether the queue is empty.

**Notes:**

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

用栈来实现队列，对比类似题目[225. Implement Stack using Queues ][225. Implement Stack using Queues]是反过来用。

class MyQueue:
        # initialize your data structure here.
        def __init__(self):
            self.A, self.B = [], []
    
        # @param x, an integer
        # @return nothing
        def push(self, x):
            self.A.append(x)
    
        # @return an integer
        def pop(self):
            self.peek()
            return self.B.pop()
    
        # @return an integer
        def peek(self):
            if not self.B:
                while self.A:
                    self.B.append(self.A.pop())
            return self.B[-1]
    
        # @return an boolean
        def empty(self):
            return not self.A and not self.B

[225. Implement Stack using Queues]: http://www.cnblogs.com/lightwindy/p/8531855.html