Linked List Cycle

Given a linked list, determine if it has a cycle in it.

判断某个链表是否有环。

方法一：

用一个hashmap来存放访问过的节点，通过比较当前节点是否存在map中来判断是否有环：

/**
 * 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) {
        Map<ListNode,Integer> map = new HashMap<ListNode,Integer>();
        while(head!=null) {
            if(!map.containsKey(head)) {
                map.put(head,1);
                head = head.next;
            }
            else return true;
        }
        return false;
    }
}

方法二：

用2个指针，慢指针一次后移一步，快指针一次后移2步，若有环则快指针会与慢指针相遇：

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