题解 | #判断链表中是否有环#

双指针fast和slow

• fast每次移动两步
• slow每次移动一步
• 如果fast能走到null,说明没有环
• 如果slow能走到和fast相等，说明有环

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
        ListNode fast = head;
        ListNode slow = head;
        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
            if (fast == null) {
                return false;
            }
            if (fast == slow) {
                return true;
            }
        }
        return false;
    }
}