练习题(六)

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.

• 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).

解答：

class Stack:

    # initialize your data structure here.
    def __init__(self):
        self.stack = []

    # @param x, an integer
    # @return nothing
    def push(self, x):
        self.stack.append(x)

    # @return nothing
    def pop(self):
        if len(self.stack) == 0:
            print ‘can not pop from an empty stack !‘
        else:
            self.stack.pop()

    # @return an integer
    def top(self):
        if len(self.stack) == 0:
            return None
        value = self.stack.pop()
        self.stack.append(value)
        return value

    # @return an boolean
    def empty(self):
        if len(self.stack) == 0:
            return True
        else:
            return False

心得：

List现在已经可以模拟stack或者sequence, 这里题目明确说明可以使用list, 所以就用list.