Leetcode 225. 用队列实现栈

题目链接

题目描述：

请你仅使用两个队列实现一个后入先出（LIFO）的栈，并支持普通栈的全部四种操作（push、top、pop 和 empty）。

原题目要求使用两个队列实现一个后入先出的栈。
但本文包括使用双队列实现栈和单队列实现栈。
分别实现了偏写入和偏读取两种方式。

1. 两个队列实现，偏写入，即push()操作为O(1)

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.poll());
        }
        Queue<Integer> temp = queue1;
        queue1 = queue2;
        queue2 = temp;
        return queue2.poll();
    }

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

    /** Returns whether the stack is empty. */
    public boolean empty() {
        return queue1.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();
 */

2. 两个队列，偏读取，即pop(),top()操作为O(1)

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);
        while (queue1.size() > 1) {
            queue2.add(queue1.poll());
        }
        while (!queue2.isEmpty()) {
            queue1.add(queue2.poll());
        }
    }

    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        return queue1.poll();
    }

    /** Get the top element. */
    public int top() {
        return queue1.peek();
    }

    /** Returns whether the stack is empty. */
    public boolean empty() {
        return queue1.isEmpty();
    }
}

3. 一个队列，偏写入

class MyStack {
    Queue<Integer> queue1;
    /** Initialize your data structure here. */
    public MyStack() {
        queue1 = 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() {
        int size = queue1.size();
        while (size-- > 1) {
            queue1.add(queue1.poll());
        }
        return queue1.poll();
    }

    /** Get the top element. */
    public int top() {
        int size = queue1.size();
        while (size-- > 1) {
            queue1.add(queue1.poll());
        }
        int x = queue1.peek();
        queue1.add(queue1.poll());
        return x;
    }

    /** Returns whether the stack is empty. */
    public boolean empty() {
        return queue1.isEmpty();
    }
}

4. 一个队列，偏读取

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

    /** Push element x onto stack. */
    public void push(int x) {
        queue1.add(x);
        int size = queue1.size();
        while (size-- > 1) {
            queue1.add(queue1.poll());
        }
    }

    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        return queue1.poll();
    }

    /** Get the top element. */
    public int top() {
        return queue1.peek();
    }

    /** Returns whether the stack is empty. */
    public boolean empty() {
        return queue1.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();
 */