Added reverse_linked_list.py

data_structures/linked_list/reverse_linked_list.py

@@ -0,0 +1,136 @@
+"""
+Linked List is described as a bunch of nodes connected to one another.
+For this scenario, this is intended for a single Linked List.
+"""
+
+from typing import Any, Optional
+
+
+class Node:
+    """
+    This class forms the backbone of a data structure called Linked List.
+    """
+
+    def __init__(self, item: Any, next: Any = None) -> None:
+        self.item = item
+        self.next = next
+
+    def __repr__(self) -> str:
+        """
+        This is here for debugging/testing purposes. This is used to convert the
+        nodes and its branch to string.
+        >>> Node(10, Node(20)).__repr__()
+        '10, 20'
+        """
+        if self.next is not None:
+            return f"{str(self.item)}, {self.next.__repr__()}"
+        else:
+            return str(self.item)
+
+
+def reverse_linked_list(input: Optional[Node]) -> Optional[Node]:
+    """
+    This function reverse the order of the given linked list.
+    >>> reverse_linked_list(Node(1, Node(2, Node(3))))
+    3, 2, 1
+    >>> reverse_linked_list(None) # it returns None
+    >>> reverse_linked_list(Node('First', Node('Second')))
+    Second, First
+    """
+    # If input is None, return None.
+    if input is None:
+        return None
+
+    # This algorithm works by popping items from input, and then pushing
+    # the popped item to output.
+
+    temp = input  # backup the input. We don't want to change the input.
+
+    # Pop the first node.
+    output = Node(temp.item)
+    temp = temp.next
+
+    # Pop data off temp and push them into output until temp has no more data.
+    while temp is not None:
+        output = Node(temp.item, output)
+        temp = temp.next
+
+    # Return the output
+    return output
+
+
+if __name__ == "__main__":
+    """
+    Running this file independently will perform assertion.
+
+    Input and Expected tests:
+    - [1, 2, 3, 4, 5] -> [5, 4, 3, 2, 1]
+    - [-9, 100, 7, 5555, 0] -> [0, 5555, 7, 100, -9]
+    - [-100, -200, -300] -> [-300, -200, -100]
+    - None -> None
+    - [10] -> [10]
+    - [10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 0] ->
+      [0, 100, 90, 80, 70, 60, 50, 40, 30, 20, 10]
+    """
+
+    import doctest
+
+    doctest.testmod()
+
+    input = [
+        Node(1, Node(2, Node(3, Node(4, Node(5))))),
+        Node(-9, Node(100, Node(7, Node(5555, Node(0))))),
+        Node(-100, Node(-200, Node(-300))),
+        None,
+        Node(10),
+        Node(
+            10,
+            Node(
+                20,
+                Node(
+                    30,
+                    Node(
+                        40,
+                        Node(
+                            50,
+                            Node(60, Node(70, Node(80, Node(90, Node(100, Node(0)))))),
+                        ),
+                    ),
+                ),
+            ),
+        ),
+    ]
+
+    expected = [
+        Node(5, Node(4, Node(3, Node(2, Node(1))))),
+        Node(0, Node(5555, Node(7, Node(100, Node(-9))))),
+        Node(-300, Node(-200, Node(-100))),
+        None,
+        Node(10),
+        Node(
+            0,
+            Node(
+                100,
+                Node(
+                    90,
+                    Node(
+                        80,
+                        Node(
+                            70,
+                            Node(60, Node(50, Node(40, Node(30, Node(20, Node(10)))))),
+                        ),
+                    ),
+                ),
+            ),
+        ),
+    ]
+
+    # Assert them here.
+    for i, e in zip(input, expected):
+        result = reverse_linked_list(i)
+        assert str(result) == str(
+            e
+        ), f"Assert Failed. Result: [{str(result)}]. Expected: [{str(e)}]"
+
+    # Indicate successful test.
+    print("Assertion for Reverse Linked List is successful.")