Hacktoberfest: Add group_odd_even_nodes algorithm to Linked Lists

DIRECTORY.md

@@ -144,6 +144,7 @@
     * [Doubly Linked List](https://github.com/TheAlgorithms/Python/blob/master/data_structures/linked_list/doubly_linked_list.py)
     * [Doubly Linked List Two](https://github.com/TheAlgorithms/Python/blob/master/data_structures/linked_list/doubly_linked_list_two.py)
     * [From Sequence](https://github.com/TheAlgorithms/Python/blob/master/data_structures/linked_list/from_sequence.py)
+    * [Group Odd Even Nodes](https://github.com/TheAlgorithms/Python/blob/master/data_structures/linked_list/group_odd_even_nodes.py)
     * [Has Loop](https://github.com/TheAlgorithms/Python/blob/master/data_structures/linked_list/has_loop.py)
     * [Is Palindrome](https://github.com/TheAlgorithms/Python/blob/master/data_structures/linked_list/is_palindrome.py)
     * [Middle Element Of Linked List](https://github.com/TheAlgorithms/Python/blob/master/data_structures/linked_list/middle_element_of_linked_list.py)

data_structures/linked_list/group_odd_even_nodes.py

@@ -0,0 +1,147 @@
+from typing import Tuple
+
+"""
+`group_odd_even_nodes` will group all odd indexed nodes at the beginning
+of the list and all even indexed nodes following the odd indexed nodes.
+
+https://www.geeksforgeeks.org/rearrange-a-linked-list-such-that-all-even-and-odd-positioned-nodes-are-together/
+
+- This will run in O(1) space and O(n) time
+- Groups by index and **NOT** the value of the node
+- Will return the head if there is one or none elements in the Linked List
+"""
+
+
+class Node:
+    def __init__(self, data=None, next=None):
+        self.data = data
+        self.next = next
+
+    def __repr__(self):
+        """Returns a visual representation of the node and all its
+        following nodes."""
+        string_rep = []
+        temp = self
+        while temp:
+            string_rep.append(f"{temp.data}")
+            temp = temp.next
+        return "->".join(string_rep)
+
+
+class LinkedList:
+    """
+    >>> linked_list = LinkedList()
+    >>> linked_list.group_odd_even_nodes()
+
+    >>> linked_list.insert(5)
+    >>> linked_list.group_odd_even_nodes()
+    5
+    >>> linked_list.insert(7)
+    >>> linked_list.group_odd_even_nodes()
+    7->5
+    >>> linked_list.insert(8)
+    >>> linked_list.group_odd_even_nodes()
+    8->5->7
+    >>> linked_list.clear_list()
+    >>> linked_list.insert(5)
+    >>> linked_list.insert(7)
+    >>> linked_list.insert(8)
+    >>> linked_list.insert(9)
+    >>> linked_list.group_odd_even_nodes()
+    9->7->8->5
+    >>> linked_list.clear_list()
+    >>> linked_list.insert(5)
+    >>> linked_list.insert(7)
+    >>> linked_list.insert(8)
+    >>> linked_list.insert(9)
+    >>> linked_list.insert(2)
+    >>> linked_list.group_odd_even_nodes()
+    2->8->5->9->7
+    """
+
+    def __init__(self):
+        self.head = None
+
+    def __iter__(self):
+        node = self.head
+
+        while node:
+            yield node.data
+            node = node.next
+
+    def __repr__(self):
+        """
+        String representation/visualization of a Linked Lists
+        """
+        return "->".join([str(item) for item in self])
+
+    def insert(self, data) -> None:
+        new_node = Node(data)
+
+        if self.is_empty():
+            self.head = new_node
+        else:
+            self.head = Node(data, self.head)
+
+    def remove(self, data) -> Tuple[str, Node]:
+        if self.is_empty():
+            return "Linked List is empty"
+        else:
+            prev, current = None, self.head
+            while current:
+                if current.data == data:
+                    if prev:
+                        prev.next = current.next
+                        current = prev
+                    else:
+                        self.head = self.head.next
+                else:
+                    prev = current
+                current = current.next
+            return self.head
+
+    def is_empty(self) -> bool:
+        return self.head is None
+
+    def group_odd_even_nodes(self) -> Node:
+        if not self.head or not self.head.next:
+            return self.head
+
+        odd = self.head
+        even = self.head.next
+
+        while even and even.next:
+            temp = even.next
+            even.next = even.next.next
+
+            temp.next = odd.next
+            odd.next = temp
+
+            odd = odd.next
+            even = even.next
+
+        return self.head
+
+    def clear_list(self):
+        self.head = None
+
+
+def main():
+    from doctest import testmod
+
+    testmod()
+
+    linked_list = LinkedList()
+    linked_list.insert(5)
+    linked_list.insert(7)
+    linked_list.insert(8)
+    linked_list.insert(9)
+    linked_list.insert(2)
+    print(linked_list)  # expect 2->9->8->7->5
+
+    linked_list.group_odd_even_nodes()
+    print(linked_list)  # expect 2->8->5->9->7
+
+
+if __name__ == "__main__":
+    main()