leetcode oj java Implement Stack using Queues

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问题描述：

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 onlypush to back,peek/pop from front, size, andis 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.

思路：用两个队列实现一个栈 q1 作为专用的队列，q2作为中转站

入栈：队列的offer() 或者add();

出栈：将q1的前q1.size()-1个依次放入q2中，poll()出q1中剩下的一个，然后再将q2的依次放入q1中。

top ：如果队列中只有一个元素，取出即可；如果队列中有多个元素，先将q1.size()-1个依次放入q2中，取出q1中剩下的，作为top值返回，再依次还原。

empty: 队列的isEmpty() 方法，

package leetcode;

import java.util.LinkedList;
import java.util.Queue;

public class MyStack {
   Queue<Integer> q1 = new LinkedList<Integer>();
   Queue<Integer> q2 = new LinkedList<Integer>();
   // Push element x onto stack.
    public void push(int x) {
        q1.offer(x);
    }

    // Removes the element on top of the stack.
    public void pop() {
       int a = q1.size();
       if(a==1){ q1.poll();}
       else{
        for(int i = 0; i<a-1; i++){
           q2.add(q1.peek());
           q1.poll();
        }
        q1.poll();
        int b = a;
        for(int i = 0; i<b-1; i++){
           q1.add(q2.peek());
           q2.poll();
        }}
    }

    // Get the top element.
    public int top() {
       int a = 0;
       if(q1.size()==1){a = q1.peek();}
       else{
       int m = q1.size();
        for(int i = 0; i<m-1; i++){
           q2.add(q1.peek());
           q1.poll();
        }
        a = q1.peek();
        q2.add(q1.peek());
        q1.poll();
        int n = q2.size();
        for(int i = 0; i<n; i++){
           q1.add(q2.peek());
           q2.poll();
        }}
//        int a = q1.peek();
        return a;
    }

    // Return whether the stack is empty.
    public boolean empty() {
        return q1.isEmpty();
    }
}

提交的状态：

16 / 16 test cases passed.

Status:

Accepted

Runtime: 110 ms

Submitted: 28 minutes ago

/----------------------------我是华丽丽的分割线--------------------------/

自己编写了主函数测试：

package leetcode;

public class Stack {

   /**
   * @param args
   */
   public static void main(String[] args) {
       // TODO Auto-generated method stub
       MyStack a = new MyStack();
       a.push(1);
       a.push(2);
        a.push(3);
       System.out.println(a.top());
    a.pop();
        System.out.println(a.top());


   }

}

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