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;
}
}
