linked list cycle

Author: dmgodoy

linkedlist/hascycle.py

@@ -0,0 +1,34 @@
+class Node():
+    def __init__(self, val: int, next: "Node" = None):
+        self.val = val
+        self.next = next
+    def __str__(self) -> str:
+        return str(self.val)
+
+def hasCycle(n: Node) -> bool:
+    slow = fast = n
+    while fast and fast.next:
+        slow = slow.next
+        fast = fast.next.next
+        if slow == fast:
+            break
+    
+    if slow != fast: return None
+
+    start = n
+    while start != slow:
+        start = start.next
+        slow = slow.next
+    return slow
+
+def printHasCycle(head: Node):
+    r1 = hasCycle(head)
+    print(f'cycle? {"starting in " + str(r1.val) if r1 else False}') #None
+
+last = Node(5)
+head = Node(1, Node(2, Node(3, Node(4, last))))
+printHasCycle(head) # False
+last.next = head
+printHasCycle(head) # Starting in 1
+
+