L225 implement-stack-using-queues.md

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. 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).

思路：

stack就是当成一个list形式的东西 给stack增加元素：stack.append() stack弹出元素，返回被弹出的元素：stack.pop() stack返回栈顶元素，注意：stack从下面开始从0计算，stack[len(self.stack)-1] 判断是否是empty stack==[] 队列是先进先出的线性表

class MyStack(object):

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.stack=[]    

    def push(self, x):
        """
        Push element x onto stack.
        :type x: int
        :rtype: void
        """
        self.stack.append(x)    

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

    def top(self):
        """
        Get the top element.
        :rtype: int
        """
        return self.stack[len(self.stack)-1]
        

    def empty(self):
        """
        Returns whether the stack is empty.
        :rtype: bool
        """
        return self.stack==[]