LeetCode： Linked List Cycle

題目鏈接：Linked List Cycle

Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space?

經典（用爛了）的追及問題。

/**
 * 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 p1 = head, p2 = head.next;
        while(p1 != null && p2 != null) {
            if(p1 == p2) {
                return true;
            } else {
                // p1走一步
                p1 = p1.next;     
                if (p2.next == null) {
                    return false;
                } else {
                    // p2走兩步
                    p2 = p2.next.next;   
                }
            }
        }
        return false;
    }
}

ps：很久沒刷題了，每天做一做，健康又益智^^。