LeetCode 循环链表判断

判断一个单链表是否循环.

解题思想，龟兔赛跑原理，如果有循环的话，乌龟和兔子总会相遇（小学数学书中的龟兔相遇问题）。

Java version

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