- \n
- Elements are inserted from the rear<\/li>\n
- Elements are removed from the front<\/li>\n
- Processing follows FIFO order<\/li>\n<\/ul>
Basic representation:<\/strong><\/p>
\nFront → 10 → 20 → 30 → Rear<\/p>\n<\/div><\/div>
Example scenario:<\/strong><\/p>
\nIf three tasks arrive in order A, B, C:<\/p>\n
Processing order will be: A → B → C<\/p>\n<\/div><\/div>
Key characteristics:<\/strong><\/p>
- \n
- FIFO structure<\/li>\n
- Ordered processing<\/li>\n
- Two pointers (front and rear)<\/li>\n
- No random access<\/li>\n
- Used for scheduling problems<\/li>\n<\/ul>
Basic array declaration:<\/strong><\/p>
\nint queue[5];
\nint front = -1;
\nint rear = -1;<\/p>\n<\/div><\/div>How Queue Works Internally<\/h2>
Queues maintain two important pointers:<\/p>
- \n
- Front pointer:<\/strong> Points to the first element.<\/li>\n
- Rear pointer:<\/strong> Points to the last inserted element.<\/li>\n<\/ul>
Example operations:<\/strong><\/p>
\nEnqueue 10
\nQueue: 10
\nFront = 0
\nRear = 0<\/p>\nEnqueue 20
\nQueue: 10 20
\nRear = 1<\/p>\nEnqueue 30
\nQueue: 10 20 30
\nRear = 2<\/p>\nDequeue<\/p>\n
Queue becomes:
\n20 30
\nFront moves to index 1.<\/p>\nImportant observation:
\nInsertion moves rear forward
\nDeletion moves front forward<\/p>\n<\/div><\/div>This is why queue insertion and deletion take O(1) time.<\/p>
Types of Queues in Data Structure<\/h2>
1. Linear Queue<\/h3>
Basic queue implementation using arrays or linked lists.<\/p>
Problem:<\/strong> After multiple deletions, empty spaces cannot be reused.<\/p>
Example issue:<\/strong><\/p>
\nQueue size = 5
\nAfter deletions:
\n_ _ 30 40 50\n<\/div><\/div>Front moved, but space was wasted.<\/p>
Used in:<\/strong><\/p>
- \n
- Simple scheduling systems<\/li>\n
- Basic buffering<\/li>\n<\/ul>
2. Circular Queue<\/h3>
A circular queue solves memory waste by connecting the last position to the first.<\/p>
Concept:<\/strong> Rear wraps around when space is available.<\/p>
Condition:<\/strong><\/p>
\nrear = (rear + 1) % size;<\/p>\n<\/div><\/div>
Used in:<\/strong><\/p>
- \n
- CPU scheduling<\/li>\n
- Streaming buffers<\/li>\n
- Network data handling<\/li>\n<\/ul>
Advantage:<\/strong> Efficient memory usage.<\/p>
3. Priority Queue<\/h3>
In a priority queue, elements are processed based on priority instead of arrival order.<\/p>
\nExample:<\/strong> Priority 1 processed before Priority 5.<\/p>\n<\/div><\/div>
Used in:<\/strong><\/p>
- \n
- Operating systems scheduling<\/li>\n
- Dijkstra algorithm<\/li>\n
- Network routing<\/li>\n<\/ul>
4. Deque (Double-Ended Queue)<\/h3>
Deque allows insertion and deletion from both ends.<\/p>
Types:<\/strong> Input restricted deque
\nOutput restricted deque<\/p>Used in:<\/strong><\/p>
- \n
- Sliding window problems<\/li>\n
- Cache implementation<\/li>\n
- Palindrome checking<\/li>\n<\/ul>
Basic Queue Operations with Coding Examples<\/h2>
1. Enqueue (Insertion)<\/h3>
Adds an element at the rear.<\/p>
\nrear++;
\nqueue[rear] = value;
\nIf queue was empty:
\nfront = rear = 0;<\/p>\n<\/div><\/div>Time complexity: O(1)<\/p>
2. Dequeue (Deletion)<\/h3>
Removes the element from the front.<\/p>
\nfront++;<\/p>\n<\/div><\/div>
If front passes rear:<\/strong> Queue becomes empty.<\/p>
Time complexity: O(1)<\/p>
3. Peek Operation<\/h3>
Returns the first element without removing it.<\/p>
\nprintf(“%d”, queue[front]);<\/p>\n<\/div><\/div>
4. Queue Underflow Condition<\/h3>
Occurs when deleting from an empty queue.<\/p>
Condition:<\/strong><\/p>
\nif(front == -1 || front > rear)<\/p>\n<\/div><\/div>
Queue is empty<\/p>
5. Queue Overflow Condition<\/h3>
Occurs when inserting into the full queue.<\/p>
Condition:<\/strong><\/p>
\nif(rear == size-1)<\/p>\n<\/div><\/div>
The queue is full<\/p>
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<\/a><\/p>When Should You Use a Queue in DSA<\/h2>
Understanding when to use a queue is important for problem-solving<\/a>.<\/p>
Use Queue when:<\/strong><\/p>
- \n
- Order of processing matters<\/li>\n
- Tasks arrive continuously<\/li>\n
- BFS traversal required<\/li>\n
- Streaming data processing<\/li>\n
- Scheduling systems<\/li>\n<\/ul>
Avoid the queue when:<\/strong><\/p>
- Rear pointer:<\/strong> Points to the last inserted element.<\/li>\n<\/ul>
- Front pointer:<\/strong> Points to the first element.<\/li>\n