LeetCode 141. Linked List Cycle题目Approach #1 (Two Pointers)代码Approach #2 (Hash Table)

/**
 * Definition for ListNode.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int val) {
 *         this.val = val;
 *         this.next = null;
 *     }
 * }
 */ 
public class Solution {
    /**
     * @param head: The first node of linked list.
     * @return: True if it has a cycle, or false
     */
    public boolean hasCycle(ListNode head) {  
        // write your code here
        if (head == null || head.next == null) {
            return false;
        }

        ListNode slow = head;
        ListNode fast = head;

    while (true) {
        if (fast == null || fast.next == null) {
            return false;    //遇到null了，说明不存在环
        }
        slow = slow.next;
        fast = fast.next.next;
        if (fast == slow) {
            return true;   //第一次相遇在Z点
        }
    }
    }
}

public boolean hasCycle(ListNode head) {
        if(head == null || head.next == null)
            return false;
        
        Set<ListNode> nodeSeen = new HashSet<ListNode>();
        while(head != null) {
            if(!nodeSeen.contains(head))
                nodeSeen.add(head);
            else
                return true;
            head = head.next;
        }
        return false;
    }