【朱迪的LeetCode刷题笔记】141. Linked List Cycle #Easy #C

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2. 141. Linked List Cycle #Easy
   
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5. Given head, the head of a linked list, determine if the linked list has a cycle in it.
   
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7. There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail's next pointer is connected to. Note that pos is not passed as a parameter.
   
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9. Return true if there is a cycle in the linked list. Otherwise, return false.
   
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13. Example 1:
    
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15. Input: head = [3,2,0,-4], pos = 1
    
16. Output: true
    
17. Explanation: There is a cycle in the linked list, where the tail connects to the 1st node (0-indexed).
    
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20. Example 2:
    
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22. Input: head = [1,2], pos = 0
    
23. Output: true
    
24. Explanation: There is a cycle in the linked list, where the tail connects to the 0th node.
    
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27. Example 3:
    
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29. Input: head = [1], pos = -1
    
30. Output: false
    
31. Explanation: There is no cycle in the linked list.
    
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34. Constraints:
    
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36. The number of the nodes in the list is in the range [0, 104].
    
37. -105 <= Node.val <= 105
    
38. pos is -1 or a valid index in the linked-list.
    
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41. Follow up: Can you solve it using O(1) (i.e. constant) memory?
    

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