Add algorithm for finding the starting node in a circular linked list.

linkedlist/find_loop.py

@@ -0,0 +1,68 @@
+"""
+Find the start of a linked list loop
+
+Some constraints:
+* It is gonna be a singly linked list.
+* The parameter is not always gonna be a circular linked list.
+* Once the loop is found, return the node.
+
+Test Cases:
+* Empty list -> None
+* Not a circular linked list -> None
+	* One element
+	* Two or more elements
+* Circular linked list general case
+"""
+class LinkedList:
+
+	def __init__(self):
+		self.root = None
+
+	# Function to insert a new node at the beginning
+	def push(self, new_data):
+		new_node = Node(new_data)
+		new_node.next = self.head
+		self.head = new_node	
+
+	def find_loop_start(self):
+		if self.root is None or self.root.next is None:
+			return None
+
+		# Detect loop using Floyd’s Cycle detection algorithm 
+		slow = fast = self.root
+		while fast.root is not None:
+			slow = slow.next
+			fast = fast.next.next
+
+			if fast is None:
+				return None
+
+			if fast == slow:
+				break
+
+		# check for the starting node
+		slow = self.root
+		while slow != fast:
+			slow = slow.next
+			fast = fast.next
+
+			if fast is None:
+				return None
+		return slow	
+
+class Node:
+
+	def __init__(self, _value):
+		self.value = _value
+		self.next = None							 
+
+
+linked_list = LinkedList()
+linked_list.push(10)
+linked_list.push(4)
+linked_list.push(15)
+linked_list.push(20)
+linked_list.push(50)
+
+# Make it a circular linked list
+linked_list.root.next.next.next.next.next = linked_list.root.next.next