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).

 

 两个栈实现队列。

1 class Queue { 2 private: 3     stack
     
       stack1,stack2; 4 public: 5     // Push element x to the back of queue. 6     void push(int x) { 7        8         stack1.push(x); 9         10     }11 12     // Removes the element from in front of queue.13     void pop(void) {14         if(stack2.empty())15         {16             while(!stack1.empty())17             {18                 stack2.push(stack1.top());19                 stack1.pop();20             }21         }22         stack2.pop();23         24     }25 26     // Get the front element.27     int peek(void) {28         if(stack2.empty())29         {30             while(!stack1.empty())31             {32                 stack2.push(stack1.top());33                 stack1.pop();34             }35         }36         return stack2.top();37         38         39     }40 41     // Return whether the queue is empty.42     bool empty(void) {43         if(stack1.empty()&&stack2.empty())44             return true;45         else46             return false;47         48     }49 };