Question

Formatted question description: https://leetcode.ca/all/225.html

 225	Implement Stack using Queues

 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 below operations are valid:
    push to back,
    peek/pop from front,
    size,
    and is empty

 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).

 @tag-stack

Algorithm

Need two queues,

1st queue: A queue is used to put the last number added to simulate the top element of the stack.
2nd queue: All the remaining numbers are placed in the other queue in order.

When the push() operation, the new number is first added to the queue of the top element of the analog stack. If there is a number in the queue at this time, the original number is put into 2nd queue, and the new number is in 1st queue. To simulate the top element of the stack.

In the top()/peek() operation, if there is a number in the top of the 1st queue, it will return directly. If there is no number, it will go to 2nd queue and take out the last number by shifting the number and add it to the queue at the top of the 1st queue.

When pop() operation, perform top()/peek() operation first to ensure that there is a number in the queue at the top of the 1st queue, and then remove the number.

When the empty() operation, when both queues are empty, the stack is empty.

Code

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

public class Implement_Stack_using_Queues {
    class MyStack {
        private Queue<Integer> queue1;
        private Queue<Integer> queue2;

        public MyStack() {
            this.queue1 = new LinkedList<Integer>();
            this.queue2 = new LinkedList<Integer>();
        }

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

        // Removes the element on top of the stack.
        public int pop() {
            while (queue1.size() > 1) {
                queue2.offer(queue1.poll());
            }

            int last = queue1.poll();

            // Swap queue1 and queue2
            Queue<Integer> temp = queue1;
            queue1 = queue2;
            queue2 = temp;

            return last;
        }

        // Get the top element.
        public int top() {
            while (queue1.size() > 1) {
                queue2.offer(queue1.poll());
            }

            // tried to re-use while part, but seems not achievable, since val is retrieved in-between 
            int ret = queue1.poll();
            queue2.offer(ret); // note: add back to q2, so q1 will always be empty

            // Swap queue 1 and queue2
            Queue<Integer> temp = queue1;
            queue1 = queue2;
            queue2 = temp;

            return ret;
        }

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

############

class MyStack {

    private Deque<Integer> q;

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

    /** Push element x onto stack. */
    public void push(int x) {
        q.offerLast(x);
        int n = q.size();
        while (n-- > 1) {
            q.offerLast(q.pollFirst());
        }
    }

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

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

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

// OJ: https://leetcode.com/problems/implement-stack-using-queues
// Time: O(N) for push, O(1) for others
// Space: O(1)
class MyStack {
    queue<int> q;
public:
    MyStack() {}
    void push(int x) {
        q.push(x);
        for (int cnt = q.size() - 1; cnt > 0; --cnt) {
            q.push(q.front());
            q.pop();
        } 
    }
    int pop() {
        int ans = q.front();
        q.pop();
        return ans;
    }
    int top() {
        return q.front();
    }
    bool empty() {
        return q.empty();
    }
};

# one queue
class Stack:

    def __init__(self):
        self._queue = collections.deque()

    def push(self, x):
        q = self._queue
        q.append(x)
        for _ in range(len(q) - 1):
            q.append(q.popleft())
        
    def pop(self):
        return self._queue.popleft()

    def top(self):
        return self._queue[0]
    
    def empty(self):
        return not len(self._queue)

# Your MyStack object will be instantiated and called as such:
# obj = MyStack()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.top()
# param_4 = obj.empty()

############

from collections import deque


class MyStack:

	def __init__(self):
		self.q1 = deque()
		self.q2 = deque()

	def push(self, x: int) -> None:
		self.q1.append(x)

	def pop(self) -> int:
		while len(self.q1) != 1:
			self.q2.append(self.q1.popleft())

		val = self.q1.popleft()
		self.q1, self.q2 = self.q2, self.q1
		return val

	def top(self) -> int:
		while len(self.q1) != 1:
			self.q2.append(self.q1.popleft())

        # tried to re-use while part, but seems not achievable, since val is retrieved in-between 
		val = self.q1[0]
		self.q2.append(self.q1.popleft())  # note: add back to q2, so q1 will always be empty

		self.q1, self.q2 = self.q2, self.q1
		return val


	def empty(self) -> bool:
		return not self.q1

# Your MyStack object will be instantiated and called as such:
# obj = MyStack()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.top()
# param_4 = obj.empty()

225 - Implement Stack using Queues

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