Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space?
/** * Definition for singly-linked list. * struct ListNode { * int val; * ListNode *next; * ListNode(int x) : val(x), next(NULL) {} * }; */ class Solution { public: bool hasCycle(ListNode *head) { if(!head) return false; ListNode * p = head; ListNode * q = head; while(q->next != NULL && q->next->next != NULL){ p = p->next; q = q->next->next; if(q == p) return true; } return false; } }; Use two pointers, walker and runner.walker moves step by step. runner moves two steps at time.if the Linked List has a cycle walker and runner will meet at some point.
