LeetCode--linked-list-cycle-ii

Given a linked list, return the node where the cycle begins. If there is no cycle, returnnull.

Follow up:
Can you solve it without using extra space?

思路：快慢指针

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode detectCycle(ListNode head) {
      ListNode p_node=null;
        if(head ==null || head.next==null){
            return null;
        }
        ListNode fast =  head;
        ListNode slow = head;
        while(fast !=null && fast.next != null ){
            //每次向前走两步
            fast=fast.next.next;
            //每次向前走一步
            slow=slow.next;
            if(slow == fast){
            //slow与fast相遇的时候
             ListNode node=head;
             while(node !=slow){
                 node=node.next;
                 slow=slow.next;
             }
             return node;
            }
            
        }
        return null;
    }
}