leetcode--Implement Stack using Queues

最新推荐文章于 2022-04-28 14:36:20 发布

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本文介绍了一种使用两个队列来模拟栈数据结构的方法。通过切换队列角色及元素转移操作，实现了栈的基本功能：压栈(push)、弹栈(pop)、获取栈顶元素(top)及判断是否为空(empty)。

mplement the following operations of a stack using queues.

push(x) -- Push element x onto stack.
pop() -- Removes the element on top of the stack.
top() -- Get the top element.
empty() -- Return whether the stack is empty.

Notes:

You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, and is empty operations are valid.
Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

Update (2015-06-11):
The class name of the Java function had been updated to MyStack instead of Stack.

[java]view plaincopy 
   
 class MyStack {  
     Queue<Integer> queue1 = new LinkedList<Integer>();  
     Queue<Integer> queue2 = new LinkedList<Integer>();  
     boolean turn = true;  
     // Push element x onto stack.  
     public void push(int x) {  
         if(turn){  
             queue1.add(x);  
         }else{  
             queue2.add(x);  
         }  
     }  
   
     // Removes the element on top of the stack.  
     public void pop() {  
         if(turn){  
             while(queue1.size()>1){  
                 queue2.add(queue1.poll());  
             }  
             queue1.poll();  
         }else{  
             while(queue2.size()>1){  
                 queue1.add(queue2.poll());  
             }  
             queue2.poll();  
         }  
         turn = !turn;  
     }  
   
     // Get the top element.  
     public int top() {  
         if(turn){  
             while(queue1.size()>1){  
                 queue2.add(queue1.poll());  
             }  
             return queue1.peek();  
         }else{  
             while(queue2.size()>1){  
                 queue1.add(queue2.poll());  
             }  
             return queue2.peek();  
         }  
     }  
   
     // Return whether the stack is empty.  
     public boolean empty() {  
         return queue1.isEmpty()&&queue2.isEmpty();  
     }  
 } 