Given a linked list, return the node where the cycle begins. If there is no cycle, return null.
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.
Note: Do not modify the linked list.
Example 1:
Input: head = [3,2,0,-4], pos = 1 Output: tail connects to node index 1 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: tail connects to node index 0 Explanation: There is a cycle in the linked list, where tail connects to the first node.Example 3:
Input: head = [1], pos = -1 Output: no cycle Explanation: There is no cycle in the linked list.
Follow up:Can you solve it without using extra space?
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这个题是不仅是判断链表中是否存在环，还要返回环的开始的位置。
用双指针，与141. Linked List Cycle 的类似，关于怎么返回这个位置，可以参考这个大佬的博客：http://www.cnblogs.com/hiddenfox/p/3408931.html
C++代码：
/** * Definition for singly-linked list. * struct ListNode { * int val; * ListNode *next; * ListNode(int x) : val(x), next(NULL) {} * }; */ class Solution { public: ListNode *detectCycle(ListNode *head) { ListNode *slow,*fast; slow = head; fast = head; while(true){ if(fast == NULL || fast->next == NULL) return NULL; slow = slow->next; fast = fast->next->next; if(slow == fast) break; } slow = head; while(slow != fast){ slow = slow->next; fast = fast->next; } return slow; } };
转载于:https://www.cnblogs.com/Weixu-Liu/p/10704334.html
