Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space?

Solution：快慢指针

思路：如果有环，则会相遇
Time Complexity: O(N) Space Complexity: O(1)

Solution Code：

public class Solution {
    public boolean hasCycle(ListNode head) {
        if(head == null) return false;
        ListNode slow = head;
        ListNode fast = head;
        
        while(runner.next != null && runner.next.next != null) {
            walker = walker.next;
            runner = runner.next.next;
            if(walker == runner) return true;
        }
        
        return false;
    }
}