225.Implement-Stack-using-Queues

225. Implement Stack using Queues

题目地址

https://leetcode.com/problems/implement-stack-using-queues/

题目描述

mplement 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).

代码

Approach 1: Using one queue. push is O(n), others are O(1).

class MyStack 
{
    Queue<Integer> queue;

    public MyStack()
    {
        this.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.
    public void pop() 
    {
        queue.poll();
    }

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

    // Return 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();
 */

Approach #2 Using two queue

push - O(1), POP O(n)

private Queue<Integer> q1 = new LinkedList<>();
private Queue<Integer> q2 = new LinkedList<>();
private int top;

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

public void pop() {
    while (q1.size() > 1) {
        top = q1.remove();
        q2.add(top);
    }
    q1.remove();
    Queue<Integer> temp = q1;
    q1 = q2;
    q2 = temp;
}

public void push(int x) {
    q2.add(x);
    top = x;
    while (!q1.isEmpty()) {                
        q2.add(q1.remove());
    }
    Queue<Integer> temp = q1;
    q1 = q2;
    q2 = temp;
}

// Removes the element on top of the stack.
public void pop() {
    q1.remove();
    if (!q1.isEmpty()) {
        top = q1.peek();
    }
}