[LeetCode] 225. Implement Stack using Queues_Easy tag: Design

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.

Example:

MyStack stack = new MyStack();

stack.push(1);
stack.push(2);
stack.top();   // returns 2
stack.pop();   // returns 2
stack.empty(); // returns false

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).

利用如下图所示的方式, 每次push的时候将queue里面的值都放到x的后面即可.

Push: O(n), others : O(1)

Code

class MyStack(object):

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.q = collections.deque()

    def push(self, x):
        """
        Push element x onto stack.
        :type x: int
        :rtype: void
        """
        size = len(self.q)
        self.q.append(x)
        while size:
            self.q.append(self.q.popleft())
            size -= 1

    def pop(self):
        """
        Removes the element on top of the stack and returns that element.
        :rtype: int
        """
        return self.q.popleft()

    def top(self):
        """
        Get the top element.
        :rtype: int
        """
        return self.q[0]

    def empty(self):
        """
        Returns whether the stack is empty.
        :rtype: bool
        """
        return not self.q

# Your MyStack object will be instantiated and called as such:
# obj = MyStack()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.top()
# param_4 = obj.empty()