leetcode---linked-list-cycle-ii---链表

Given a linked list, return the node where the cycle begins. If there is no cycle, returnnull.

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:
ListNode *detectCycle(ListNode *head)
{
if(!head)
return head;
ListNode *fast = head;
ListNode *slow = head;
while(fast->next && fast->next->next)
{
fast = fast->next->next;
slow = slow->next;
if(fast->val == slow->val)
{
slow = head;
while(fast && slow && fast->val != slow->val)
{
fast = fast->next;
slow = slow->next;
}
return slow;
}
}
return NULL;
}
};