622. Design Circular Queue

CDesign your implementation of the circular queue. The circular queue is a linear data structure in which the operations are performed based on FIFO (First In First Out) principle and the last position is connected back to the first position to make a circle. It is also called "Ring Buffer".

One of the benefits of the circular queue is that we can make use of the spaces in front of the queue. In a normal queue, once the queue becomes full, we cannot insert the next element even if there is a space in front of the queue. But using the circular queue, we can use the space to store new values.

Your implementation should support following operations:

MyCircularQueue(k): Constructor, set the size of the queue to be k.
Front: Get the front item from the queue. If the queue is empty, return -1.
Rear: Get the last item from the queue. If the queue is empty, return -1.
enQueue(value): Insert an element into the circular queue. Return true if the operation is successful.
deQueue(): Delete an element from the circular queue. Return true if the operation is successful.
isEmpty(): Checks whether the circular queue is empty or not.
isFull(): Checks whether the circular queue is full or not.

Example:

MyCircularQueue circularQueue = new MyCircularQueue(3); // set the size to be 3
circularQueue.enQueue(1);  // return true
circularQueue.enQueue(2);  // return true
circularQueue.enQueue(3);  // return true
circularQueue.enQueue(4);  // return false, the queue is full
circularQueue.Rear();  // return 3
circularQueue.isFull();  // return true
circularQueue.deQueue();  // return true
circularQueue.enQueue(4);  // return true
circularQueue.Rear();  // return 4

Solution

Approach 1: use two pointers to maintain the front and end positions. Can also use a front pointer, then a length variable.

Code (Python)

Approach 1:

class MyCircularQueue:

    def __init__(self, k: int):
        """
        Initialize your data structure here. Set the size of the queue to be k.
        """
        # idea: use start and end pointers, or a start and length pointer
        self._queue = [None] * k # assume all inserted items != None
        self._start = 0 # the index of the first item
        self._end = 0 # the index of the last item + 1

    def enQueue(self, value: int) -> bool:
        """
        Insert an element into the circular queue. Return true if the operation is successful.
        """
        if self.isFull():
            return False
        self._queue[self._end] = value
        self._end = (self._end + 1) % len(self._queue)
        return True

    def deQueue(self) -> bool:
        """
        Delete an element from the circular queue. Return true if the operation is successful.
        """
        if self.isEmpty():
            return False
        self._queue[self._start] = None
        self._start = (self._start + 1) % len(self._queue)
        return True

    def Front(self) -> int:
        """
        Get the front item from the queue.
        """
        if self.isEmpty():
            return -1
        return self._queue[self._start]

    def Rear(self) -> int:
        """
        Get the last item from the queue.
        """
        if self.isEmpty():
            return -1
        return self._queue[self._end - 1]

    def isEmpty(self) -> bool:
        """
        Checks whether the circular queue is empty or not.
        """
        return self._start == self._end and self._queue[self._start] == None

    def isFull(self) -> bool:
        """
        Checks whether the circular queue is full or not.
        """
        return self._start == self._end and self._queue[self._start] != None

Code (C++)

Approach 1:

Approach 2: