linked-list-cycle-ii (链表判环 并返回交点)

题目描述

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?

/** * 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) {        if(head == null || head.next == null) {            return null;        }        ListNode fast = head, slow = head;        while(fast != null && fast.next != null) {            fast = fast.next.next;            slow = slow.next;            if(fast == slow) {                break;            }        }        if(fast == null || fast.next == null) {            return null;        }        fast = head;        while(fast != slow) {            fast = fast.next;            slow = slow.next;        }        return fast;    }}