Add solutions for 141 Linked List Cycle

LeetCode/0141_Linked_List_Cycle.py

@@ -0,0 +1,39 @@
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, x):
+#         self.val = x
+#         self.next = None
+
+class Solution:
+    '''
+    Solution using Floyd's cycle-finding algorithm (tortoise and hare)
+    '''
+
+    def hasCycle(self, head: ListNode) -> bool:
+        if not head or not head.next:
+            return False
+
+        fast = slow = head
+        while fast and fast.next:
+            slow = slow.next
+            fast = fast.next.next
+
+            # If there is a cycle, fast will inevitably be on the same node as
+            # slow.
+            if fast is slow:
+                return True
+        return False
+
+
+class SolutionO1:
+    '''
+    Solution to follow up. Disregards contents of each ListNode.
+    '''
+
+    def hasCycle(self, head: ListNode) -> bool:
+        while head:
+            if head.val is None:
+                return True
+            head.val = None
+            head = head.next
+        return False