LeetCode - 141. Linked List Cycle

题目：

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

思路与步骤：

采用“快慢指针”查检查链表是否含有环。让一个指针一次走一步，另一个一次走两步，如果链表中含有环，快指针会再次和慢指针相遇。

编程实现:

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

注意循环条件！否则会有异常

while(fast.next!=null && fast.next.next!=null)