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?
注意两个测试用例，head->head和head->NULL

/**
* 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 *fast = head, *slow = head;
while( fast && fast->next){
fast = fast->next->next;
slow = slow->next;
if( fast == slow){
fast = head;
while( fast != slow){
fast = fast->next;
slow = slow->next;
}
return fast;
}
}

return NULL;
}
};