https://leetcode-cn.com/problems/linked-list-cycle-ii/solution/linked-list-cycle-ii-kuai-man-zhi-zhen-shuang-zhi-/
数学推导，
/**
* 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 fast=head,slow=head;
while(true){
if(fast==null||fast.next==null) return null;
fast=fast.next.next;
slow=slow.next;
if(fast==slow) break;
}
fast=head;
while(fast!=slow){
fast=fast.next;
slow=slow.next;
}
return slow;
}
}
