232. 用栈实现队列

class MyQueue {
    private:

    stack<int> m_input;

    stack<int> m_output;

public:
    /** Initialize your data structure here. */
    MyQueue() {

    }

    /** Push element x to the back of queue. */
    void push(int x) {
        m_input.push(x);
    }

    /** Removes the element from in front of queue and returns that element. */
    int pop() {

      int result=peek();
      m_output.pop();
      return result;
    }

    /** Get the front element. */
    int peek() {

      if ( m_output.empty()) 
      {
          while ( !m_input.empty())
          {
              m_output.push(m_input.top());
              m_input.pop();
          }
      }

      return   m_output.top();
    }

    /** Returns whether the queue is empty. */
    bool empty() {

        return m_input.empty() && m_output.empty();
    }
};