Circular Queue - Amazon Top Interview Questions


Problem Statement :


Implement a circular queue with the following methods:

CircularQueue(int capacity) which creates an instance of a circular queue with size capacity. 

Circular queues are implemented using an array which holds the enqueued values with pointers pointing to the start and end of the queue.
When the queue reaches the end of the array, it will start to fill items from the start of the array if they were dequeued.
boolean enqueue(int val) which adds val to the queue if it has space.
If the queue is full, return false to denote it can't be added and return true otherwise.
boolean dequeue() which removes the first element that was enqueued. 
If the queue is empty, return false to denote it can't be removed and return true otherwise.
int front() which returns the first element that was enqueued. If the queue is empty, return -1.
int top() which returns the last element that was enqueued. If the queue is empty, return -1.
boolean isFull() which returns whether the queue has capacity elements.
boolean isEmpty() which returns whether the queue has no elements.

Constraints

0 ≤ n ≤ 100,000 where n is the number of calls to the queue

Example 1

Input

methods = ["constructor", "enqueue", "enqueue", "enqueue", "top", "front", "isFull", "isEmpty", "dequeue", "enqueue", "dequeue", "dequeue", "dequeue", "isEmpty"]
arguments = [[3], [1], [2], [3], [], [], [], [], [], [4], [], [], [], []]`

Output

[None, True, True, True, 3, 1, True, False, True, True, True, True, True, True]

Explanation

q = CircularQueue(3)

q.enqueue(1)
q.enqueue(2)
q.enqueue(3)
q.top() == 3
q.front() == 1
q.isFull() == True
q.isEmpty() == False
q.dequeue() == True
q.enqueue(4) == True
q.dequeue() == True
q.dequeue() == True
q.dequeue() == True
q.isEmpty() == True



Solution :



title-img




                        Solution in C++ :

class CircularQueue {
    public:
    vector<int> arr;
    int i, j, n, total;
    CircularQueue(int capacity) {
        arr.resize(capacity);
        i = j = total = 0;
        n = capacity;
    }
    bool enqueue(int val) {
        if (isFull()) return false;
        total += 1;
        arr[i] = val;
        i = (i + 1) % n;
        return true;
    }

    bool dequeue() {
        if (isEmpty()) return false;
        j = (j + 1) % n;
        total -= 1;
        return true;
    }
    int front() {
        if (isEmpty()) return -1;
        return arr[j];
    }
    int top() {
        if (isEmpty()) return -1;
        return arr[(i - 1 + n) % n];
    }

    bool isFull() {
        return total == n ? true : false;
    }

    bool isEmpty() {
        return total == 0 ? true : false;
    }
};
                    




                        Solution in Python : 
                            
class CircularQueue:
    def __init__(self, capacity):
        self.arr = [0] * capacity
        self.n = capacity
        self.front_idx = 0
        self.total = 0

    def enqueue(self, val):
        if self.isFull():
            return False
        self.arr[(self.front_idx + self.total) % self.n] = val
        self.total += 1
        return True

    def dequeue(self):
        if self.isEmpty():
            return False
        self.front_idx += 1
        self.front_idx %= self.n
        self.total -= 1
        return True

    def front(self):
        if self.isEmpty():
            return -1
        return self.arr[self.front_idx]

    def top(self):
        if self.isEmpty():
            return -1
        return self.arr[(self.front_idx + self.total - 1) % self.n]

    def isFull(self):
        return self.total == self.n

    def isEmpty(self):
        return self.total == 0
                    


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