141. Linked List Cycle

问题
Given head, the head of a linked list, determine if the linked list has a cycle in it.

There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail’s next pointer is connected to. Note that pos is not passed as a parameter.

Return true if there is a cycle in the linked list. Otherwise, return false.

遍历并移动快慢指针。
如两个指针最终相遇，则链表中有循环。
如快指针移动到链表尾部，则链表无循环。

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