leetcode--Linked List Cycle II

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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：

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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;    }};

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