problem: 0142 linked list cycle II

0142 Linked List Cycle II/0142 Linked List Cycle II.md

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+# 0142. Linked List Cycle II
+
+* Difficulty: medium
+* Link: https://leetcode.com/problems/linked-list-cycle-ii/
+* Topics: Linked-List, Multiple-Pointers
+
+# Clarification
+
+1. Check the inputs and outputs
+    - INPUT: Linked List
+    - OUTPUT: ListNode
+2. Check the main goal
+    - find entry point
+
+# Naive Solution
+
+### Thought Process
+
+1. slow, fast pointer to find the meet point
+2. two pointer start from meet point and beginning 
+3. the meet point is the entry point
+- Implement
+
+    ```python
+    # Definition for singly-linked list.
+    # class ListNode:
+    #     def __init__(self, x):
+    #         self.val = x
+    #         self.next = None
+
+    class Solution:
+        def detectCycle(self, head: Optional[ListNode]) -> Optional[ListNode]:
+            # find cycle
+            slow = head
+            fast = head
+            while slow and fast and fast.next:
+                slow = slow.next
+                fast = fast.next.next
+                if slow == fast:
+                    break
+
+            if slow == None or fast == None or fast.next == None:
+                return None
+
+            # find meet point
+            fast = head
+            while slow != fast:
+                slow = slow.next
+                fast = fast.next
+
+            return slow
+    ```
+
+
+### Complexity
+
+- Time complexity: $O(n)$
+- Space complexity:$O(1)$
+
+# Note
+
+- Related
+
+    [0**287. Find the Duplicate Number** ](https://www.notion.so/0287-Find-the-Duplicate-Number-1f3d3e7e0ab74f22b4402128fc8351ba)