225. Implement Stack using Queues (E)

Implement Stack using Queues (E)

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).

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题意

使用队列Queue来实现栈Stack。要求只能使用队列的add/offer、remove/poll、peek、size、isEmpty方法。

思路

可使用双队列或单队列，直接上代码。

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代码实现 - 双队列

class MyStack {

    private Queue<Integer> q1, q2;
    private int top;

    /** Initialize your data structure here. */
    public MyStack() {
        q1 = new LinkedList<>();
        q2 = new LinkedList<>();
    }

    /** Push element x onto stack. */
    public void push(int x) {
        q1.offer(x);
        top = x;
    }

    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        while (q1.size() > 1) {
            top = q1.poll();
            q2.offer(top);
        }
        int res = q1.poll();
        Queue<Integer> temp = q1;
        q1 = q2;
        q2 = temp;

        return res;
    }

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

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

代码实现 - 单队列

class MyStack {

    private Queue<Integer> q;

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

    /** Push element x onto stack. */
    public void push(int x) {
        int size = q.size();
        q.offer(x);
        while (size != 0) {
            q.offer(q.poll());
            size--;
        }
    }

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

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

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