LeetCode No.225 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.

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).

Update (2015-06-11):

The class name of the Java function had been updated to MyStack instead of Stack.

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题目链接：https://leetcode.com/problems/implement-stack-using-queues/

题目大意：用队列来实现栈的基本操作。

思路：使用双向队列。

参考代码：

class Stack {public:    // Push element x onto stack.    void push(int x) {        dq.push_back ( x ) ;    }    // Removes the element on top of the stack.    void pop() {        if ( ! dq.empty() )            dq.pop_back() ;    }    // Get the top element.    int top() {        return dq.back() ;    }    // Return whether the stack is empty.    bool empty() {        return dq.empty() ;    }private:    deque <int> dq ;};