用数组实现栈 用栈实现队列

#!/usr/bin/python
# -*- coding: utf-8 -*-
class A:
    arr=[]
    @classmethod
    def push(cls,i):
        cls.arr=[i]+cls.arr
    @classmethod
    def pop(cls):
        ret=cls.arr[0]
        cls.arr=cls.arr[1:]
        return ret
A.arr=[66,661,662,663]
for i in range(10):
    A.push(i)
    import pprint
    pprint.pprint(A.arr)
    ret=A.pop()
    pprint.pprint(A.arr)

　　

小结：

1、

借助linkedlist，每次添加元素后，反转，取逆序

Implement Stack using Queues - LeetCode
https://leetcode.com/problems/implement-stack-using-queues/solution/

Implement Stack using Queues - LeetCode Articles
https://leetcode.com/articles/implement-stack-using-queues/

使用队列实现栈的下列操作：

• push(x) -- 元素 x 入栈
• pop() -- 移除栈顶元素
• top() -- 获取栈顶元素
• empty() -- 返回栈是否为空

注意:

• 你只能使用队列的基本操作-- 也就是 push to back, peek/pop from front, size, 和 is empty 这些操作是合法的。
• 你所使用的语言也许不支持队列。 你可以使用 list 或者 deque（双端队列）来模拟一个队列 , 只要是标准的队列操作即可。
• 你可以假设所有操作都是有效的（例如, 对一个空的栈不会调用 pop 或者 top 操作）。

Approach #1 (Two Queues, push - O(1), pop O(n))

Approach #2 (Two Queues, push - O(n), pop O(1) )

Approach #3 (One Queue, push - O(n), pop O(1))

package leetcode;

import java.util.LinkedList;
import java.util.Queue;

class MyStack {

    //one Queue solution
    private Queue<Integer> q = new LinkedList<Integer>();

    public static void main(String[] args) {
        MyStack myStack = new MyStack();
        myStack.push(-2);
        myStack.push(0);
        myStack.push(-3);
        myStack.push(13);
        myStack.pop();
        myStack.top();
    }

    // Push element x onto stack.
    public void push(int x) {
        q.add(x);
        for (int i = 1; i < q.size(); i++) { //rotate the queue to make the tail be the head
            q.add(q.poll());
        }
    }

    // Removes the element on top of the stack.
    public int pop() {
        return q.poll();
    }

    // Get the top element.
    public int top() {
        return q.peek();
    }

    // Return whether the stack is empty.
    public boolean empty() {
        return q.isEmpty();
    }
}

class MyStack:

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

    def push(self, x: int) -> None:
        """
        Push element x onto stack.
        """
        
        self.myqueue=[x]+self.myqueue
        

    def pop(self) -> int:
        """
        Removes the element on top of the stack and returns that element.
        """
        ret=self.myqueue[0]
        self.myqueue=self.myqueue[1:]
        return ret

        

    def top(self) -> int:
        """
        Get the top element.
        """
        ret=self.myqueue[0]
        return ret

    def empty(self) -> bool:
        """
        Returns whether the stack is empty.
        """
        return len(self.myqueue)==0
                


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