leetcode:用两个栈实现一个队列 Implement Queue using Stacks

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

Notes:

• You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is empty operations are valid.
• Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.

• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

  class MyQueue {public:    /** Initialize your data structure here. */    MyQueue() {            }        /** Push element x to the back of queue. */    void push(int x) {        while(!st1.empty())        {            st2.push(st1.top());            st1.pop();        }        st1.push(x);        while(!st2.empty())        {            st1.push(st2.top());            st2.pop();        }    }        /** Removes the element from in front of queue and returns that element. */    int pop() {      int top=st1.top();      st1.pop();      return top;    }        /** Get the front element. */    int peek() {        return st1.top();;    }        /** Returns whether the queue is empty. */    bool empty() {        return st1.empty();    }private:    stack<int> st1;    stack<int> st2;};/** * Your MyQueue object will be instantiated and called as such: * MyQueue obj = new MyQueue(); * obj.push(x); * int param_2 = obj.pop(); * int param_3 = obj.peek(); * bool param_4 = obj.empty(); */