add solution: linked-list-cycle

Author: dusunax

linked-list-cycle/dusunax.py

@@ -0,0 +1,33 @@
+'''
+# 141. Linked List Cycle
+
+use two pointers, Floyd's Tortoise and Hare algorithm
+
+> Tortoise and Hare algorithm  
+>- slow pointer moves one step at a time
+>- fast pointer moves two steps at a time
+>- if there is a cycle, slow and fast will meet at some point
+>- if there is no cycle, fast will reach the end of the list
+
+## Time Complexity: O(n)
+In the worst case, we need to traverse the entire list to determine if there is a cycle.
+
+## Space Complexity: O(1)
+no extra space is used, only the two pointers.
+'''
+class Solution:
+    def hasCycle(self, head: Optional[ListNode]) -> bool:
+        if not head or not head.next:
+            return False
+        
+        slow = head
+        fast = head
+
+        while fast and fast.next:
+            slow = slow.next
+            fast = fast.next.next
+
+            if slow == fast:
+                return True
+        
+        return False