LeetCode:Linked List Cycle II

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

null

.

Note: Do not modify the linked list.

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) {
ListNode *slow = head,*fast=head;

while(fast&&fast->next)
{
slow=slow->next;
fast=fast->next->next;

if(slow==fast)
{
ListNode *slow2=head;
while(slow2!=slow)
{
slow2=slow2->next;
slow=slow->next;

}
return slow2;
}
}
return NULL;

}
};