add a new reverse-linked-list

data_structures/linked_list/reverse_linked_list.py

@@ -0,0 +1,58 @@
+"""
+https://www.enjoyalgorithms.com/blog/reverse-linked-list
+"""
+
+
+class ListNode:
+    """Definition for singly-linked list."""
+
+    def __init__(self, val=0, next=None):
+        self.val = val
+        self.next = next
+
+
+def reverse_linked_list(head: ListNode) -> ListNode:
+    """
+    Reverse a singly linked list.
+
+    Args:
+        head: The head node of the linked list.
+
+    Returns:
+        The new head node of the reversed linked list.
+
+    Examples:
+        >>> a = ListNode(1)
+        >>> b = ListNode(2)
+        >>> c = ListNode(3)
+        >>> a.next, b.next = b, c
+        >>> head = reverse_linked_list(a)
+        >>> [head.val, head.next.val, head.next.next.val]
+        [3, 2, 1]
+    """
+    prev = None
+    current = head
+    while current:
+        nxt = current.next
+        current.next = prev
+        prev = current
+        current = nxt
+    return prev
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()
+
+    # Example execution
+    a = ListNode(1)
+    b = ListNode(2)
+    c = ListNode(3)
+    a.next, b.next = b, c
+
+    print("Original Linked List: 1 -> 2 -> 3")
+    new_head = reverse_linked_list(a)
+    print(
+        f"Reversed Linked List: {new_head.val} -> {new_head.next.val} -> {new_head.next.next.val}"
+    )