LeetCode 225 Implement Stack using Queues

Posted 2020-08-06

Implement 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).

 

思路：

基本上还是通过两个队列去实现，在实现栈的出栈操作的时候，将主队列中的元素出队至备用队列中，将队尾元素留下并弹出，再将备用队列中的元素出队至主队列中即可。而返回栈顶元素的方法大同小异，区别仅在于是否需要将队尾元素删除。

 

解法：

 1 import java.util.Queue;
 2 
 3 public class MyStack
 4 {
 5     private Queue<Integer> mainQueue;
 6     private Queue<Integer> tempQueue;
 7 
 8     public MyStack()
 9     {
10         mainQueue = new LinkedList<>();
11         tempQueue = new LinkedList<>();
12     }
13 
14     public void push(int x)
15     { mainQueue.offer(x); }
16 
17     public void pop()
18     {
19         while(mainQueue.size() > 1)
20             tempQueue.offer(mainQueue.poll());
21         mainQueue.poll();
22         while(!tempQueue.isEmpty())
23             mainQueue.offer(tempQueue.poll());
24     }
25 
26     public int top()
27     {
28         while(mainQueue.size() > 1)
29             tempQueue.offer(mainQueue.poll());
30         int topValue = mainQueue.poll();
31         tempQueue.offer(mainQueue.poll());
32         while(!tempQueue.isEmpty())
33             mainQueue.offer(tempQueue.poll());
34         return topValue;
35     }
36 
37     public boolean empty()
38     { return mainQueue.isEmpty(); }
39 }