Added poll method in Singly LinkedList

Author: abimurugan

src/main/java/com/thealgorithms/datastructures/lists/SinglyLinkedList.java

@@ -334,6 +334,17 @@ public void insertNth(int data, int position) {
         size++;
     }
 
+    /**
+     * @return delete the head node and returns it's value
+     */
+    public int poll() {
+        if (head == null) return -1;
+
+        int headValue = head.value;
+        head = head.next;
+        return headValue;
+    }
+
     /**
      * Deletes a node at the head
      */