Create merge_two_lists.py that implements merging of two sorted linked lists (#3874)

* Create merge_two_lists.py that implements merging of two sorted linked lists

* Update merge_two_lists.py

Fixed formatting errors

* Fixed trailing whitespace

* Change name of function to def __str__()

* updating DIRECTORY.md

* Imported classes from singly_linked_list.py

* Update merge_two_lists.py

* Update merge_two_lists.py

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: Christian Clauss <cclauss@me.com>

DIRECTORY.md

@@ -153,6 +153,7 @@
     * [From Sequence](https://github.com/TheAlgorithms/Python/blob/master/data_structures/linked_list/from_sequence.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)
+    * [Merge Two Lists](https://github.com/TheAlgorithms/Python/blob/master/data_structures/linked_list/merge_two_lists.py)
     * [Middle Element Of Linked List](https://github.com/TheAlgorithms/Python/blob/master/data_structures/linked_list/middle_element_of_linked_list.py)
     * [Print Reverse](https://github.com/TheAlgorithms/Python/blob/master/data_structures/linked_list/print_reverse.py)
     * [Singly Linked List](https://github.com/TheAlgorithms/Python/blob/master/data_structures/linked_list/singly_linked_list.py)

data_structures/linked_list/merge_two_lists.py

@@ -0,0 +1,83 @@
+"""
+Algorithm that merges two sorted linked lists into one sorted linked list.
+"""
+from __future__ import annotations
+
+from collections.abc import Iterable, Iterator
+from dataclasses import dataclass
+from typing import Optional
+
+test_data_odd = (3, 9, -11, 0, 7, 5, 1, -1)
+test_data_even = (4, 6, 2, 0, 8, 10, 3, -2)
+
+
+@dataclass
+class Node:
+    data: int
+    next: Optional[Node]
+
+
+class SortedLinkedList:
+    def __init__(self, ints: Iterable[int]) -> None:
+        self.head: Optional[Node] = None
+        for i in reversed(sorted(ints)):
+            self.head = Node(i, self.head)
+
+    def __iter__(self) -> Iterator[int]:
+        """
+        >>> tuple(SortedLinkedList(test_data_odd)) == tuple(sorted(test_data_odd))
+        True
+        >>> tuple(SortedLinkedList(test_data_even)) == tuple(sorted(test_data_even))
+        True
+        """
+        node = self.head
+        while node:
+            yield node.data
+            node = node.next
+
+    def __len__(self) -> int:
+        """
+        >>> for i in range(3):
+        ...     len(SortedLinkedList(range(i))) == i
+        True
+        True
+        True
+        >>> len(SortedLinkedList(test_data_odd))
+        8
+        """
+        return len(tuple(iter(self)))
+
+    def __str__(self) -> str:
+        """
+        >>> str(SortedLinkedList([]))
+        ''
+        >>> str(SortedLinkedList(test_data_odd))
+        '-11 -> -1 -> 0 -> 1 -> 3 -> 5 -> 7 -> 9'
+        >>> str(SortedLinkedList(test_data_even))
+        '-2 -> 0 -> 2 -> 3 -> 4 -> 6 -> 8 -> 10'
+        """
+        return " -> ".join([str(node) for node in self])
+
+
+def merge_lists(
+    sll_one: SortedLinkedList, sll_two: SortedLinkedList
+) -> SortedLinkedList:
+    """
+    >>> SSL = SortedLinkedList
+    >>> merged = merge_lists(SSL(test_data_odd), SSL(test_data_even))
+    >>> len(merged)
+    16
+    >>> str(merged)
+    '-11 -> -2 -> -1 -> 0 -> 0 -> 1 -> 2 -> 3 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8 -> 9 -> 10'
+    >>> list(merged) == list(sorted(test_data_odd + test_data_even))
+    True
+    """
+    return SortedLinkedList(list(sll_one) + list(sll_two))
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()
+    SSL = SortedLinkedList
+    print(merge_lists(SSL(test_data_odd), SSL(test_data_even)))