Update 0142.环形链表II.md

problems/0142.环形链表II.md

@@ -221,25 +221,55 @@ public class Solution {
 Python：
 
 ```python
+（版本一）快慢指针法
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, x):
+#         self.val = x
+#         self.next = None
+
+
 class Solution:
     def detectCycle(self, head: ListNode) -> ListNode:
-        slow, fast = head, head
+        slow = head
+        fast = head
+        
         while fast and fast.next:
             slow = slow.next
             fast = fast.next.next
-            # 如果相遇
+            
+            # If there is a cycle, the slow and fast pointers will eventually meet
             if slow == fast:
-                p = head
+                # Move one of the pointers back to the start of the list
-                q = slow
+                slow = head
-                while p!=q:
+                while slow != fast:
-                    p = p.next
+                    slow = slow.next
-                    q = q.next
+                    fast = fast.next
-                #你也可以return q
+                return slow
-                return p
+        # If there is no cycle, return None
+        return None
+```
+```python
+（版本二）集合法
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, x):
+#         self.val = x
+#         self.next = None
+
+
+class Solution:
+    def detectCycle(self, head: ListNode) -> ListNode:
+        visited = set()
+        
+        while head:
+            if head in visited:
+                return head
+            visited.add(head)
+            head = head.next
         
         return None
 ```
-
 Go：
 
 ```go