232. 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 Queue {public:    // Push element x to the back of queue.    stack<int> s1;    stack<int> s2;    void push(int x)     {        s1.push(x);    }    // Removes the element from in front of queue.    void pop(void)     {        if(!s2.empty())            s2.pop();        else        {            while(!s1.empty())            {                s2.push(s1.top());                s1.pop();            }            s2.pop();        }    }    // Get the front element.    int peek(void)     {         if(!s2.empty())            return s2.top();        else        {            while(!s1.empty())            {                s2.push(s1.top());                s1.pop();            }            return s2.top();        }    }    // Return whether the queue is empty.    bool empty(void)     {        return s1.empty()&&s2.empty();    }};