Given a linked list, return the node where the cycle begins. If there is no cycle, return null
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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) { // IMPORTANT: Please reset any member data you declared, as // the same Solution instance will be reused for each test case. if (head==null) { return null; } ListNode pFast = head; ListNode pSlow = head; boolean first = true; while (pFast != null) { if (pSlow == pFast && !first) { //从相遇点找入口 ListNode p = head; while (p!=null) { if(p==pFast) { return p; } p = p.next; pFast = pFast.next; } } pSlow = pSlow.next; if (pFast.next !=null) { pFast = pFast.next.next; } else { pFast = null; } first = false; } return null; } }