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?

/**
* 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;
ListNode* fast = head;
if(head == NULL || head-> next == NULL)
return false;
while(fast != NULL && fast->next!= NULL)
{
slow = slow->next;
fast = fast->next->next;
if(slow == fast)
{
slow = head;
while(slow != fast)
{
slow = slow->next;
fast = fast->next;
}
return slow;
}
}
return NULL;

}

};

Round 2:

class Solution {
public:
ListNode *detectCycle(ListNode *head) {
ListNode *p1 = head, *p2 = head;
while(p2 != NULL)
{
p2 = p2->next;
p1 = p1->next;
if(p2)
p2 = p2->next;
else
return NULL;
if(p1 == p2)
break;
}
if(p1 == p2 && p2 != NULL)
{
p1 = head;
while(p1 != p2)
{
p1 = p1->next;
p2 = p2->next;
}
return p1;
}
return NULL;
}
};