/*
 * @lc app=leetcode id=141 lang=java
 *
 * [141] Linked List Cycle
 */

// @lc code=start
/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    /**
     * 快慢指针，若链表中存在环，则快指针一次动两步，慢指针一次动一步，快指针一定会赶上慢指针
     * 
     * @param head
     * @return
     */
    public boolean hasCycle(ListNode head) {
        ListNode slow = head, fast = head;
        boolean isCycle = false;
        while(fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
            if(fast == slow) {
                isCycle = true;
                break;
            }
        }
        return isCycle;
    }
}
// @lc code=end