leetcode--Linked List Cycle II
Given a linked list, return the node where the cycle begins. If there is no cycle, return null
.
Follow up:
Can you solve it without using extra space?
算法:
java:
/** * Definition for singly-linked list. * class ListNode { * int val; * ListNode next; * ListNode(int x) { * val = x; * next = null; * } * } */ public class Solution { public ListNode detectCycle(ListNode head) { if(null==head||null==head.next){ return null; } ListNode p,q,s,join=null; p=head; q=head; s=head; while(q!=null){ p=p.next; q=q.next; if(q!=null) q=q.next; else{ return null; } if(p==q){ join=q; break; } } if(join!=null){ while(join!=s){ join=join.next; s=s.next; } return s; } return null; } }
c++:
/** * 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) { if(!head||!head->next) return NULL; ListNode *p,*q,*s,*start; p=head; q=head; s=NULL; while(q){ q=q->next; p=p->next; if(q){ q=q->next; }else{ return NULL; } if(p==q){ s=p; break; } } if(s){ start=head; while(s!=start){ s=s->next; start=start->next; } return start; } return NULL; } };