141. Linked List Cycle

题目141. Linked List Cycle

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

思路:使用快慢指针, fast每次跳两个节点,low一次跳一个节点,如果fast或者low最后为null,说明
链表没有节点. 如果fast和low相等,那么说明链表存在环

public class Solution {
    public boolean hasCycle(ListNode head) {
        if(head == null){
            return false;
        }
        
        ListNode lowerNode = head;
        ListNode fastNode = head;
        while(lowerNode != null && fastNode != null){
            if(fastNode.next == null){
                return false;
            }
            fastNode = fastNode.next.next;
            lowerNode = lowerNode.next;
            if(lowerNode == fastNode){
                return true;
            }
        }
        return false;
    }
}