Determining whether the circular linked list,

Given a list, the list is determined whether a ring.

To show the list given ring, we use an integer  pos to indicate a position connected to the end of the list of the list (starting with index 0). If  pos is  -1, there is no loop on this list.

Speed ​​pointer method can be used to solve the problem

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
bool hasCycle(struct ListNode *head) {
    struct ListNode *slow = NULL;
    struct ListNode *fast = NULL;
    
    if (head == NULL || head->next == NULL)
        return false;
    
    slow = head;
    fast = head;
    while(fast->next && fast->next->next){
        slow = slow->next;
        fast = fast->next->next;
        if (slow == fast)
            return true;
    }
    
    return false;
    
}