Contains loops.py add (#2442)

* added an algorithm which checks a linked list for loops and returns true if one is found

* added doctests and clarified meaning of loop

* Define Node.__iter__()

* Update and rename has_loop.py to has_duplicate_data.py

* Update has_duplicate_data.py

* Update has_duplicate_data.py

* Update and rename has_duplicate_data.py to has_loop.py

* Update has_loop.py

Co-authored-by: Christian Clauss <cclauss@me.com>

data_structures/linked_list/has_loop.py

@@ -0,0 +1,60 @@
+from typing import Any
+
+
+class ContainsLoopError(Exception):
+    pass
+
+
+class Node:
+    def __init__(self, data: Any) -> None:
+        self.data = data
+        self.next_node = None
+
+    def __iter__(self):
+        node = self
+        visited = []
+        while node:
+            if node in visited:
+                raise ContainsLoopError
+            visited.append(node)
+            yield node.data
+            node = node.next_node
+
+    @property
+    def has_loop(self) -> bool:
+        """
+        A loop is when the exact same Node appears more than once in a linked list.
+        >>> root_node = Node(1)
+        >>> root_node.next_node = Node(2)
+        >>> root_node.next_node.next_node = Node(3)
+        >>> root_node.next_node.next_node.next_node = Node(4)
+        >>> root_node.has_loop
+        False
+        >>> root_node.next_node.next_node.next_node = root_node.next_node
+        >>> root_node.has_loop
+        True
+        """
+        try:
+            list(self)
+            return False
+        except ContainsLoopError:
+            return True
+
+
+if __name__ == "__main__":
+    root_node = Node(1)
+    root_node.next_node = Node(2)
+    root_node.next_node.next_node = Node(3)
+    root_node.next_node.next_node.next_node = Node(4)
+    print(root_node.has_loop)  # False
+    root_node.next_node.next_node.next_node = root_node.next_node
+    print(root_node.has_loop)  # True
+
+    root_node = Node(5)
+    root_node.next_node = Node(6)
+    root_node.next_node.next_node = Node(5)
+    root_node.next_node.next_node.next_node = Node(6)
+    print(root_node.has_loop)  # False
+
+    root_node = Node(1)
+    print(root_node.has_loop)  # False