[Leetcode]141. 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?

思路：快慢指针，设置一个走两步的快指针，和一个走一步的慢指针，如果有环，则它们一定会相遇。

/**
 * 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) {
        if (head==null)
            return false;
        ListNode p1 = head,p2 = head.next;
        while (p2!=null){
            p2 = p2.next;
            if (p2==null)
                return false;
            p2 = p2.next;
            p1 = p1.next;
            if (p2==p1)
                return true;
        }
        return false;
    }
}