Linked List cycle: determines whether a linked list has a loop)

Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?

Note: The linked list loop is not the same as the tail pointer and header pointer, but may form a loop in the middle section. Therefore, you cannot only determine whether the element and the first element overlap.

Set two pointers: p_fast and p_slow. From the beginning, in the traversal chain table, p_fast takes two nodes each time, while p_slow takes one node each time. If there is a loop, the two pointers must overlap:

 1 /** 2  * Definition for singly-linked list. 3  * struct ListNode { 4  *     int val; 5  *     ListNode *next; 6  *     ListNode(int x) : val(x), next(NULL) {} 7  * }; 8  */ 9 class Solution {10 public:11     bool hasCycle(ListNode *head) {12         if(head == NULL) return false;13         14         ListNode *p_fast = head;15         ListNode *p_slow = head;16         17         do{18             p_slow = p_slow->next;19             if(p_fast != NULL)20                 p_fast = p_fast->next;21             if(p_fast != NULL)22                 p_fast = p_fast->next;23             else24                 return false;25         }while(p_fast != p_slow);26         27         return true;28         29     }30 };

 

Linked List cycle: determines whether a linked list has a loop)