LinkedList - 141. Linked List Cycle

141. Linked List Cycle

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

To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.

Example 1:

Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the second node.

Follow up:
Can you solve it using O(1) (i.e. constant) memory?

思路：

链表找环，两种做法，一种是用hash表来记录是否有走过这个节点，走过就是有环，第二种做法是快慢指针，快指针一次走两步，慢指针一次走一步，如果没有环，快指针先到达链表尾，如果有环，则快指针先进入环内转圈，等慢指针也进入环内，两个指针就一定会相遇。注意快指针是走两步，要避免空针的情况，比如当前fast指针走到倒数第二位，再走两步就会报空针。

代码：

java：

/**

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