[LeetCode] Implement Stack using Queues

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).

     这道题就不说了，和之前说过的那道implement Queue using Stack的思路差不多。

     还是建立两个Queue之间互相转换就好，记得所写的算法运行后必须保证两个queue的其中一个队列为空。

     我的答案这里把push写成了offer然后pop写成了poll，不过leetcode还是识别了哈哈。

     因为我写的时候有参考java platform standard（http://docs.oracle.com/javase/7/docs/api/java/util/Queue.html#peek()）这里面还是写的offer/poll所以就……。

class MyStack {  LinkedList<Integer> a=new LinkedList<Integer>();  LinkedList<Integer> b=new LinkedList<Integer>();  // Push element x onto stack.  public void push(int x) {    if(a.size()==0){      a.offer(x);    }else{      if(a.size()>0){        b.offer(x);        int size=a.size();        while(size>0){          b.offer(a.poll());          size--;        }      }else if(b.size()>0){        a.offer(x);        int size=b.size();        while(size>0){          a.offer(b.poll());          size--;        }      }          }  }  // Removes the element on top of the stack.   public void pop() {     if(a.size()>0){      a.poll();     }else if(b.size()>0){       b.poll();     }      }  // Get the top element.i  public int top() {    if(a.size()>0){      return a.peek();    }else if(b.size()>0){      return b.peek();    }    return 0;  }  // Return whether the stack is empty.  public boolean empty() {    return a.isEmpty()&b.isEmpty();  }}

原标题：[LeetCode] Implement Stack using Queues

关键词：