LeetCode 225. Implement Stack using Queues

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解答1[Java]:使用两个队列

核心思想

使用两个队列 q1q2,push 的时候 push 到 q1 中,pop的时候,先把 q1 的 n-1 个元素 push 到 q2 中,剩下最后一个元素,然后弹出这个元素并返回。然后交换 q1 和 q2 的引用。

代码

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

class MyStack {
    Queue<Integer> queue1;
    Queue<Integer> queue2;

    /** Initialize your data structure here. */
    public MyStack() {
        queue1 = new LinkedList<>();
        queue2 = new LinkedList<>();
    }

    /** Push element x onto stack. */
    public void push(int x) {
        queue1.add(x);
    }

    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        while (queue1.size() > 1) {
            queue2.add(queue1.remove());
        }
        Queue<Integer> temp = queue1;
        queue1 = queue2;
        queue2 = temp;
        return queue2.remove();
    }

    /** Get the top element. */
    public int top() {
        while (queue1.size() > 1) {
            queue2.add(queue1.remove());
        }
        int topElement = queue1.peek();
        queue2.add(queue1.remove());
        Queue<Integer> temp = queue1;
        queue1 = queue2;
        queue2 = temp;
        return topElement;
    }

    /** Returns whether the stack is empty. */
    public boolean empty() {
        if (queue1.isEmpty() && queue2.isEmpty()) {
            return true;
        } else {
            return false;
        }
    }
}

/**
 * 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();
 */

解答2[Java]:使用一个队列

核心思想

push 的时候,先把元素加到最后,然后自己弹出元素的同时让元素进入队列,循环n-1次,这样刚刚push进来的元素就变到了第一个。

代码

public class MyStack {
    Queue<Integer> queue;

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

    /** 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();
 */

参考资料

Java队列Queue接口详解

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