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Author: hogan-tech

0142-linked-list-cycle-ii/0142-linked-list-cycle-ii.py

@@ -0,0 +1,53 @@
+# time complexity: O(n)
+# space complexity: O(n)
+from typing import Optional
+
+
+class ListNode:
+    def __init__(self, x):
+        self.val = x
+        self.next = None
+
+
+class Solution:
+    def detectCycle(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        nodeSeen = set()
+        node = head
+        while node:
+            if node in nodeSeen:
+                return node
+            nodeSeen.add(node)
+            node = node.next
+        return None
+
+# time complexity: O(n)
+# space complexity: O(1)
+class Solution:
+    def detectCycle(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        slow = head
+        fast = head
+
+        while fast and fast.next:
+            slow = slow.next
+            fast = fast.next.next
+
+            if slow == fast:
+                break
+
+        if not fast or not fast.next:
+            return None
+
+        fast = head
+
+        while slow != fast:
+            slow = slow.next
+            fast = fast.next
+
+        return slow
+
+head = ListNode(3)
+head.next = ListNode(2)
+head.next.next = ListNode(0)
+head.next.next.next = ListNode(-4)
+head.next.next.next.next = head.next
+print(Solution().detectCycle(head))