Leetcode 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.

用栈来实现队列的操作，在这里使用两个栈，保持一个栈为空，一个非空。我们知道栈是先进后出的，所以我们对于队列的push操作，首先将栈s1的元素入到s2中，然后将要入队列的元素x入s1中，接下来将s2中的元素入s1中，这样保持后来的元素存在栈底，先来的元素在栈顶，接下来的获得队列首元素和出队列操作直接出栈和获得非空栈顶元素即可。

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