Circular Queue

Posted: 14 Feb, 2021
Difficulty: Easy

PROBLEM STATEMENT

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You will be given ‘Q’ queries. You need to implement a circular queue according to those queries. Each query will belong to one of these two types:

1 ‘X’: Enqueue element ‘X’ into the end of the nth queue. Returns true if the element is enqueued, otherwise false.

2: Dequeue the element at the front of the nth queue. Returns -1 if the queue is empty, otherwise, returns the dequeued element.
Note:
Enqueue means adding an element to the end of the queue, while Dequeue means removing the element from the front of the queue.
Input Format:
The first line of input contains two space-separated integers ‘N’ and ‘Q’ denoting the size of queue and number of queries, respectively. 

The next ‘Q’ lines specify the type of operation/query to be performed on the data structure.

Each query contains an integer ‘P’ denoting the type of query.

For query of type 1, the integer ‘P’ is equal to 1 and it is followed by one integer ‘X’ denoting the element on which operation is to be performed.

For query of type 2, the integer ‘P’ is equal to 2 which dequeues the element.
Output Format:
For each query, return the output returned after performing the corresponding operation on the data structure. 

Print the output of each test case in a separate line.
Note:
You don’t need to print anything. It has already been taken care of. You just have to complete the given functions.
Constraints:
1 <= N <= 1000
1 <= Q <= 10^5 
1 <= P <= 2
1 <= X <= 10^5

Time limit: 1 sec
Approach 1

In this approach, we will be implementing a circular queue using arrays. A circular queue has two key methods or purpose:

 

  1. enqueue():
    • Check whether the queue is full.
      • A queue is full when the front is next to the rear. For example, with a queue of size 6, if front is 0 and rear is 5, or if front is 2 and rear is 1, it means that the queue is full.
      • If it is full, then return false.
      • If the queue is not full, then check if rear is the last index.
        • If it is, set rear to 0;
        • If it is not, increment rear and add the value at that index.
  2. dequeue():
    • Check whether the queue is empty (i.e., if front/rear has a value of -1).
      • If it is empty, the return -1.
      • If the queue is not empty, then check if the queue has only one value (i.e., front == rear).
        • If it does have only one value, set both rear and front to -1.
        • If it does not, check if front is the last index of the queue and, if so, set front to 0, otherwise, increment front.
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