LeetCode225：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 emptyoperations 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).

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LeetCode：链接

用两个队列实现栈，队列从头出，也就是只能pop(0)！！！

因为还有其他的操作，所以用辅助栈移动完之后，再移动回主栈。那么只有主栈有值，辅助栈用完就是空的。

我们先往栈内压入一个元素a，接下来继续往栈内压入b,c两个元素。我们把它们都插入queue1。这个时候 queue1包含3个元素a,b,c其中a位于队列的头部，c位于队列的尾部。

现在我们考虑从栈内弹出一个元素。根据栈的后入先出的原则，最后被压入栈的c应该最先被弹出。由于c位于queue1的尾部，而我们每次只能从队列的头部删除元素，因此我们可以从queue1中依次删除a/b并插入到queue2中，再从queue1中删除c。这就相当于从栈中弹出元素c了。我们可以用同样的方法从栈内弹出元素b。

top操作就在主栈操作，直接append在栈之后。

class MyStack(object):

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.queue1 = []
        self.queue2 = []

    def push(self, x):
        """
        Push element x onto stack.
        :type x: int
        :rtype: void
        """
        self.queue1.append(x)
        
    def pop(self):
        """
        Removes the element on top of the stack and returns that element.
        :rtype: int
        """
        # copy queue1 to queue2, except the last in Queue
        for i in range(len(self.queue1)-1):
            self.queue2.append(self.queue1.pop(0))
        # move all back to queue1
        for i in range(len(self.queue2)):
            self.queue1.append(self.queue2.pop(0))
        return self.queue1.pop(0)
        
    def top(self):
        """
        Get the top element.
        :rtype: int
        """
        # 只用了一个队列实现
        for i in range(len(self.queue1)):
            top = self.queue1.pop(0)
            self.queue1.append(top)
        return top

    def empty(self):
        """
        Returns whether the stack is empty.
        :rtype: bool
        """
        return not self.queue1

# 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()