Given a linked list, determine if it has a cycle in it.
To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.
Example 1:
Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the second node.
Example 2:
Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the first node.
Example 3:
Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.
Follow up:
Can you solve it without using extra space? (O(1) (i.e. constant) memory)?
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 *fast = head; 15 ListNode *slow = head; 16 17 while(true){ 18 // If a node is NULL, there is no cycle. 19 if(fast->next == NULL || fast->next->next == NULL) return false; 20 21 slow = slow->next; 22 fast = fast->next->next; 23 24 // When the fast and the slow run into the same node, there‘s a cycle. 25 if ( slow->val == fast->val ) 26 return true; 27 } 28 } 29 };
[Algorithm] 11. Linked List Cycle
原文：https://www.cnblogs.com/jjlovezz/p/10307664.html