Added Double Ended Queue In C

Queue/Double Ended Queue/Double_Ended_Queue.c

@@ -0,0 +1,164 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+#define MAX 5
+
+int deque[MAX];
+int front = -1;
+int rear = -1;
+
+int isFull()
+{
+    return ((front == 0 && rear == MAX - 1) || (front == rear + 1));
+}
+
+int isEmpty()
+{
+    return (front == -1);
+}
+
+void insertFront(int key)
+{
+    if (isFull())
+    {
+        printf("Overflow: Unable to insert element at the front. Deque is full.\n");
+        return;
+    }
+
+    if (front == -1)
+    {
+        front = 0;
+        rear = 0;
+    }
+    else if (front == 0)
+    {
+        front = MAX - 1;
+    }
+    else
+    {
+        front = front - 1;
+    }
+
+    deque[front] = key;
+    printf("Inserted %d at the front.\n", key);
+}
+
+void insertRear(int key)
+{
+    if (isFull())
+    {
+        printf("Overflow: Unable to insert element at the rear. Deque is full.\n");
+        return;
+    }
+
+    if (rear == -1)
+    {
+        front = 0;
+        rear = 0;
+    }
+    else if (rear == MAX - 1)
+    {
+        rear = 0;
+    }
+    else
+    {
+        rear = rear + 1;
+    }
+
+    deque[rear] = key;
+    printf("Inserted %d at the rear.\n", key);
+}
+
+void deleteFront()
+{
+    if (isEmpty())
+    {
+        printf("Underflow: Unable to delete element from the front. Deque is empty.\n");
+        return;
+    }
+
+    int removed = deque[front];
+
+    if (front == rear)
+    {
+        front = -1;
+        rear = -1;
+    }
+    else if (front == MAX - 1)
+    {
+        front = 0;
+    }
+    else
+    {
+        front = front + 1;
+    }
+
+    printf("Deleted %d from the front.\n", removed);
+}
+
+void deleteRear()
+{
+    if (isEmpty())
+    {
+        printf("Underflow: Unable to delete element from the rear. Deque is empty.\n");
+        return;
+    }
+
+    int removed = deque[rear];
+
+    if (front == rear)
+    {
+        front = -1;
+        rear = -1;
+    }
+    else if (rear == 0)
+    {
+        rear = MAX - 1;
+    }
+    else
+    {
+        rear = rear - 1;
+    }
+
+    printf("Deleted %d from the rear.\n", removed);
+}
+
+void displayDeque()
+{
+    if (isEmpty())
+    {
+        printf("Deque is empty.\n");
+        return;
+    }
+
+    printf("Deque elements are: ");
+    int i = front;
+    while (1)
+    {
+        printf("%d ", deque[i]);
+        if (i == rear)
+            break;
+        i = (i + 1) % MAX;
+    }
+    printf("\n");
+}
+
+int main()
+{
+    insertRear(5);
+    displayDeque();
+
+    insertFront(15);
+    displayDeque();
+
+    insertRear(25);
+    displayDeque();
+
+    deleteFront();
+    displayDeque();
+
+    deleteRear();
+    displayDeque();
+
+    return 0;
+}

Queue/Double Ended Queue/README.md

@@ -0,0 +1,56 @@
+# Representation
+
+In a deque we maintain two pointers front and rear, that point to either end of the deque. The elements in a deque extend from the front end to the rear end and since it is circular, dequeue[n–1] is followed by dequeue[0].
+
+# Insertion at Front in Deque in C
+
+To insert an element at the front end of the deque first, check if the deque is full or not if deque is not full then follow the below approach:
+
+Approach:
+
+First, check the position of the front in our array.
+If front < 1 , reinitialize front as the last index of the array i.e. front = N-1.
+Then, add new element to array[front].
+
+# Insertion at Rear in Deque in C
+
+To insert an element at the rear end of the deque, follow the below approach:
+
+Approach:
+
+First, check if the deque is full or not.
+If the deque is full , reinitialize rear with 0 (rear = 0) else increase rear by 1.
+Then, add the element to array[rear].
+
+# Deletion at Front in Deque in C
+
+To delete an element at the front end of the deque first, follow the below approach:
+
+Approach:
+
+First, check if deque is empty or not.
+If the deque is empty (front == -1), then we cannot perform deletion operation. In this condition, we will simply print undeflow.
+If deque contains only 1 element (front = rear) , then only one deletion operation can be performed. set front = -1 and rear = -1.
+Else if the front is at the last index ( front == n-1 ) , set front at starting index of deque (front = 0).
+If none of the above case exists, just increment front by 1 (front = front + 1).
+
+# Deletion at Rear in Deque in C
+
+To delete an element at the rear end of the deque, follow the below approach:
+
+First, check if the deque is empty or not.
+If the deque is empty (front = -1), then deletion operation cannot be performed and we will print underflow.
+If the deque has only 1 element( front==rear), we will set front = -1 and rear =-1.
+If the rear is at the starting index of deque (rear == 0) , then set rear to last index (rear = n-1).
+If none of the above case exists, just decrement rear by 1 (rear = rear-1).
+
+# Check if Deque is Empty or Not Empty
+
+To check if the deque is empty simply check the front if front = -1, then deque is empty else it is not empty.
+
+# Check if Deque is Full or Not Full
+
+To check if the deque is full simply check the below conditions:
+
+If front == 0 and rear == n-1 , then deque is Full
+If front == rear + 1 , then also deque is Full.