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).

• 题目解析：
• 用堆栈stack实现队列，仅仅能使用堆栈的操作，push()，pop（）。empty().

• 方法：
• 用两个堆栈实现。代码例如以下：

• class Queue {public:    stack
         
           st1;    stack
          
            st2;    bool st1_use=1;      bool st2_use=0;      // Push element x to the back of queue.    void push(int x) {        if(st1_use==1)        st1.push(x);        else if(st2_use==1)        st2.push(x);    }    // Removes the element from in front of queue.    void pop(void) {        if(st1.empty()&&st2.empty())          exit(0);        if(st1_use==1&&st2_use==0)        {            while(st1.size()>1)            {                int temp=st1.top();                st2.push(temp);                st1.pop();            }            st1.pop();			while(st2.size()>0)            {                int temp=st2.top();                st1.push(temp);                st2.pop();            }            st1_use==1;            st2_use==0;			return;        }        if(st1_use==0&&st2_use==1)        {            while(st2.size()>1)            {                int temp=st2.top();                st1.push(temp);                st2.pop();            }            st2.pop();			while(st1.size()>0)            {                int temp=st1.top();                st2.push(temp);                st1.pop();            }            st1_use==0;            st2_use==1;			return;        }    }    // Get the front element.    int peek(void) {        if(st1_use==1&&st2_use==0)        {			int temp;            while(st1.size()>0)            {                temp=st1.top();                st2.push(temp);                st1.pop();            }						while(st2.size()>0)            {                int temp1=st2.top();                st1.push(temp1);                st2.pop();            }            st1_use==1;            st2_use==0;			return temp;        }        if(st1_use==0&&st2_use==1)        {			int temp;            while(st2.size()>0)            {                int temp=st2.top();                st1.push(temp);                st2.pop();            }						while(st1.size()>0)            {                int temp1=st1.top();                st2.push(temp1);                st1.pop();            }            st1_use==0;            st2_use==1;			return temp;        }    }    // Return whether the queue is empty.    bool empty(void) {        if(st1_use==1&&st1.empty())        return true;        if(st2_use==1&&st2.empty())        return true;        return false;    }};