ImplementStackusingQueues.java

package easy;

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

/**
 * 
 * ClassName: ImplementStackusingQueues
 * @author chenyiAlone  
 * Create Time: 2019/02/10 20:34:07
 * Description: No.225
 *
 * ------------ update -------------
 * 思路:
 *
 *      更正了以前的题解，之前的题解使用了临时数组来保存从 `queue` 中 `poll()` 的值
 *      每次 `offer(x)` 的值只需要向后移动 `queue.size() - 1` 个位置
 *      (因为这里的 queue已经将 `x` 入队了，所以移动未加入之前的 `size` 即可)
 *
 *
 * 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.
 * Example:
 *
 * MyStack stack = new MyStack();
 *
 * stack.push(1);
 * stack.push(2);
 * stack.top();   // returns 2
 * stack.pop();   // returns 2
 * stack.empty(); // returns false
 * 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).
 *
 *
 *
 */
public class ImplementStackusingQueues {
    private Queue<Integer> queue;
    /** Initialize your data structure here. */
    public ImplementStackusingQueues() {
        queue = new LinkedList<>();
    }

    /** Push element x onto stack. */
    public void push(int x) {
        queue.offer(x);
        for (int i = 0; i < queue.size() - 1; i++) {
            queue.offer(queue.poll());
        }
    }

    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        return queue.poll();
    }

    /** Get the top element. */
    public int top() {
        return queue.peek();
    }

    /** Returns whether the stack is empty. */
    public boolean empty() {
        return queue.isEmpty();
    }


    private class ImplementStackusingQueuesOld {
        LinkedList<Integer> queue;

        /**
         * Initialize your data structure here.
         */
        public ImplementStackusingQueuesOld() {
            queue = new LinkedList<>();
        }

        /**
         * Push element x onto stack.
         */
        public void push(int x) {
            queue.add(x);
            for (int i = 0; i < queue.size() - 1; i++) {
                queue.add(queue.poll());
            }
        }

        /**
         * Removes the element on top of the stack and returns that element.
         */
        public int pop() {
            return queue.poll();
        }

        /**
         * Get the top element.
         */
        public int top() {
            return queue.peek();
        }

        /**
         * Returns whether the stack is empty.
         */
        public boolean empty() {
            return queue.isEmpty();
        }
    }
}

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack obj = new MyStack();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.top();
 * boolean param_4 = obj.empty();
 */