102. Linked List Cycle【medium】

Given a linked list, determine if it has a cycle in it.

 

Example

Given -21->10->4->5, tail connects to node index 1, return true

Challenge Follow up:
Can you solve it without using extra space?

解法一：

class Solution {
public:
    /**
     * @param head: The first node of linked list.
     * @return: True if it has a cycle, or false
     */
    bool hasCycle(ListNode *head) {
        ListNode * fast, * slow;
        if (head == NULL) {
            return false;
        }
        slow = head;
        fast = head->next;
        
        while (fast != NULL && fast->next != NULL) {
            if (slow == fast) {
                return true;
            }
            slow = slow->next;
            fast = fast->next->next;
        }
        
        return false;
    }
};