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

力扣中国上有一篇非常棒的解题思路：142. Linked List Cycle II