Skip to content

622. Design Circular Queue

Description

Design 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.

Implement the MyCircularQueue class:

  • MyCircularQueue(k) Initializes the object with the size of the queue to be k.
  • int Front() Gets the front item from the queue. If the queue is empty, return -1.
  • int Rear() Gets the last item from the queue. If the queue is empty, return -1.
  • boolean enQueue(int value) Inserts an element into the circular queue. Return true if the operation is successful.
  • boolean deQueue() Deletes an element from the circular queue. Return true if the operation is successful.
  • boolean isEmpty() Checks whether the circular queue is empty or not.
  • boolean isFull() Checks whether the circular queue is full or not.

You must solve the problem without using the built-in queue data structure in your programming language. 

 

Example 1:

Input
["MyCircularQueue", "enQueue", "enQueue", "enQueue", "enQueue", "Rear", "isFull", "deQueue", "enQueue", "Rear"]
[[3], [1], [2], [3], [4], [], [], [], [4], []]
Output
[null, true, true, true, false, 3, true, true, true, 4]

Explanation
MyCircularQueue myCircularQueue = new MyCircularQueue(3);
myCircularQueue.enQueue(1); // return True
myCircularQueue.enQueue(2); // return True
myCircularQueue.enQueue(3); // return True
myCircularQueue.enQueue(4); // return False
myCircularQueue.Rear();     // return 3
myCircularQueue.isFull();   // return True
myCircularQueue.deQueue();  // return True
myCircularQueue.enQueue(4); // return True
myCircularQueue.Rear();     // return 4

 

Constraints:

  • 1 <= k <= 1000
  • 0 <= value <= 1000
  • At most 3000 calls will be made to enQueue, deQueue, Front, Rear, isEmpty, and isFull.

Solutions

Solution 1: Array Simulation

We can use an array $q$ of length $k$ to simulate a circular queue, with a pointer $\textit{front}$ to record the position of the front element. Initially, the queue is empty, and $\textit{front}$ is $0$. Additionally, we use a variable $\textit{size}$ to record the number of elements in the queue, initially $\textit{size}$ is $0$.

When calling the enQueue method, we first check if the queue is full, i.e., $\textit{size} = k$. If it is full, we return $\textit{false}$. Otherwise, we insert the element at position $(\textit{front} + \textit{size}) \bmod k$, then $\textit{size} = \textit{size} + 1$, indicating that the number of elements in the queue has increased by $1$. Finally, we return $\textit{true}$.

When calling the deQueue method, we first check if the queue is empty, i.e., $\textit{size} = 0$. If it is empty, we return $\textit{false}$. Otherwise, we set $\textit{front} = (\textit{front} + 1) \bmod k$, indicating that the front element has been dequeued, then $\textit{size} = \textit{size} - 1$.

When calling the Front method, we first check if the queue is empty, i.e., $\textit{size} = 0$. If it is empty, we return $-1$. Otherwise, we return $q[\textit{front}]$.

When calling the Rear method, we first check if the queue is empty, i.e., $\textit{size} = 0$. If it is empty, we return $-1$. Otherwise, we return $q[(\textit{front} + \textit{size} - 1) \bmod k]$.

When calling the isEmpty method, we simply check if $\textit{size} = 0$.

When calling the isFull method, we simply check if $\textit{size} = k$.

In terms of time complexity, the above operations all have a time complexity of $O(1)$. The space complexity is $O(k)$.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
class MyCircularQueue:

    def __init__(self, k: int):
        self.q = [0] * k
        self.size = 0
        self.capacity = k
        self.front = 0

    def enQueue(self, value: int) -> bool:
        if self.isFull():
            return False
        self.q[(self.front + self.size) % self.capacity] = value
        self.size += 1
        return True

    def deQueue(self) -> bool:
        if self.isEmpty():
            return False
        self.front = (self.front + 1) % self.capacity
        self.size -= 1
        return True

    def Front(self) -> int:
        return -1 if self.isEmpty() else self.q[self.front]

    def Rear(self) -> int:
        if self.isEmpty():
            return -1
        return self.q[(self.front + self.size - 1) % self.capacity]

    def isEmpty(self) -> bool:
        return self.size == 0

    def isFull(self) -> bool:
        return self.size == self.capacity


# Your MyCircularQueue object will be instantiated and called as such:
# obj = MyCircularQueue(k)
# param_1 = obj.enQueue(value)
# param_2 = obj.deQueue()
# param_3 = obj.Front()
# param_4 = obj.Rear()
# param_5 = obj.isEmpty()
# param_6 = obj.isFull()

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
class MyCircularQueue {
    private int[] q;
    private int front;
    private int size;
    private int capacity;

    public MyCircularQueue(int k) {
        q = new int[k];
        capacity = k;
    }

    public boolean enQueue(int value) {
        if (isFull()) {
            return false;
        }
        int idx = (front + size) % capacity;
        q[idx] = value;
        ++size;
        return true;
    }

    public boolean deQueue() {
        if (isEmpty()) {
            return false;
        }
        front = (front + 1) % capacity;
        --size;
        return true;
    }

    public int Front() {
        if (isEmpty()) {
            return -1;
        }
        return q[front];
    }

    public int Rear() {
        if (isEmpty()) {
            return -1;
        }
        int idx = (front + size - 1) % capacity;
        return q[idx];
    }

    public boolean isEmpty() {
        return size == 0;
    }

    public boolean isFull() {
        return size == capacity;
    }
}

/**
 * Your MyCircularQueue object will be instantiated and called as such:
 * MyCircularQueue obj = new MyCircularQueue(k);
 * boolean param_1 = obj.enQueue(value);
 * boolean param_2 = obj.deQueue();
 * int param_3 = obj.Front();
 * int param_4 = obj.Rear();
 * boolean param_5 = obj.isEmpty();
 * boolean param_6 = obj.isFull();
 */

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
class MyCircularQueue {
private:
    int front;
    int size;
    int capacity;
    vector<int> q;

public:
    MyCircularQueue(int k) {
        capacity = k;
        q = vector<int>(k);
        front = size = 0;
    }

    bool enQueue(int value) {
        if (isFull()) return false;
        int idx = (front + size) % capacity;
        q[idx] = value;
        ++size;
        return true;
    }

    bool deQueue() {
        if (isEmpty()) return false;
        front = (front + 1) % capacity;
        --size;
        return true;
    }

    int Front() {
        if (isEmpty()) return -1;
        return q[front];
    }

    int Rear() {
        if (isEmpty()) return -1;
        int idx = (front + size - 1) % capacity;
        return q[idx];
    }

    bool isEmpty() {
        return size == 0;
    }

    bool isFull() {
        return size == capacity;
    }
};

/**
 * Your MyCircularQueue object will be instantiated and called as such:
 * MyCircularQueue* obj = new MyCircularQueue(k);
 * bool param_1 = obj->enQueue(value);
 * bool param_2 = obj->deQueue();
 * int param_3 = obj->Front();
 * int param_4 = obj->Rear();
 * bool param_5 = obj->isEmpty();
 * bool param_6 = obj->isFull();
 */

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
type MyCircularQueue struct {
    front    int
    size     int
    capacity int
    q        []int
}

func Constructor(k int) MyCircularQueue {
    q := make([]int, k)
    return MyCircularQueue{0, 0, k, q}
}

func (this *MyCircularQueue) EnQueue(value int) bool {
    if this.IsFull() {
        return false
    }
    idx := (this.front + this.size) % this.capacity
    this.q[idx] = value
    this.size++
    return true
}

func (this *MyCircularQueue) DeQueue() bool {
    if this.IsEmpty() {
        return false
    }
    this.front = (this.front + 1) % this.capacity
    this.size--
    return true
}

func (this *MyCircularQueue) Front() int {
    if this.IsEmpty() {
        return -1
    }
    return this.q[this.front]
}

func (this *MyCircularQueue) Rear() int {
    if this.IsEmpty() {
        return -1
    }
    idx := (this.front + this.size - 1) % this.capacity
    return this.q[idx]
}

func (this *MyCircularQueue) IsEmpty() bool {
    return this.size == 0
}

func (this *MyCircularQueue) IsFull() bool {
    return this.size == this.capacity
}

/**
 * Your MyCircularQueue object will be instantiated and called as such:
 * obj := Constructor(k);
 * param_1 := obj.EnQueue(value);
 * param_2 := obj.DeQueue();
 * param_3 := obj.Front();
 * param_4 := obj.Rear();
 * param_5 := obj.IsEmpty();
 * param_6 := obj.IsFull();
 */

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
class MyCircularQueue {
    private queue: number[];
    private left: number;
    private right: number;
    private capacity: number;

    constructor(k: number) {
        this.queue = new Array(k);
        this.left = 0;
        this.right = 0;
        this.capacity = k;
    }

    enQueue(value: number): boolean {
        if (this.isFull()) {
            return false;
        }
        this.queue[this.right % this.capacity] = value;
        this.right++;
        return true;
    }

    deQueue(): boolean {
        if (this.isEmpty()) {
            return false;
        }
        this.left++;
        return true;
    }

    Front(): number {
        if (this.isEmpty()) {
            return -1;
        }
        return this.queue[this.left % this.capacity];
    }

    Rear(): number {
        if (this.isEmpty()) {
            return -1;
        }
        return this.queue[(this.right - 1) % this.capacity];
    }

    isEmpty(): boolean {
        return this.right - this.left === 0;
    }

    isFull(): boolean {
        return this.right - this.left === this.capacity;
    }
}

/**
 * Your MyCircularQueue object will be instantiated and called as such:
 * var obj = new MyCircularQueue(k)
 * var param_1 = obj.enQueue(value)
 * var param_2 = obj.deQueue()
 * var param_3 = obj.Front()
 * var param_4 = obj.Rear()
 * var param_5 = obj.isEmpty()
 * var param_6 = obj.isFull()
 */

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
struct MyCircularQueue {
    q: Vec<i32>,
    size: usize,
    capacity: usize,
    front: usize,
}

impl MyCircularQueue {
    fn new(k: i32) -> Self {
        MyCircularQueue {
            q: vec![0; k as usize],
            size: 0,
            capacity: k as usize,
            front: 0,
        }
    }

    fn en_queue(&mut self, value: i32) -> bool {
        if self.is_full() {
            return false;
        }
        let rear = (self.front + self.size) % self.capacity;
        self.q[rear] = value;
        self.size += 1;
        true
    }

    fn de_queue(&mut self) -> bool {
        if self.is_empty() {
            return false;
        }
        self.front = (self.front + 1) % self.capacity;
        self.size -= 1;
        true
    }

    fn front(&self) -> i32 {
        if self.is_empty() {
            -1
        } else {
            self.q[self.front]
        }
    }

    fn rear(&self) -> i32 {
        if self.is_empty() {
            -1
        } else {
            let rear = (self.front + self.size - 1) % self.capacity;
            self.q[rear]
        }
    }

    fn is_empty(&self) -> bool {
        self.size == 0
    }

    fn is_full(&self) -> bool {
        self.size == self.capacity
    }
}

Comments