Linked List Cycle II

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

Example

Challenge

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