141. 环形链表

141. 环形链表

设置一个fast指针，一个slow指针，fast一次走两步，slow一次走一步。如果fast和slow相遇，则说明有环。反之没相遇则无环。

注意快慢指针的while循环条件是fast.next != null && fast.next.next != null

/**
 * 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) {
        if(head == null) return false;
        ListNode fast = head, slow = head;
        while(fast.next!=null && fast.next.next!=null){
            fast = fast.next.next;
            slow = slow.next;
            if(fast == slow) return true;
        }
        return false;
    }
}