# LeetCode-142. Linked List Cycle II

## LeetCode-142. Linked List Cycle II

Posted by Jae on September 11, 2019

### 1、题目

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

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.

Note: Do not modify the linked list.

Example 1:

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


Example 2:

Input: head = [1,2], pos = 0
Output: tail connects to node index 0
Explanation: There is a cycle in the linked list, where tail connects to the first node.


Example 3:

Input: head = [1], pos = -1
Output: no cycle
Explanation: There is no cycle in the linked list.


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

### 2、思路

2(a + b) = a + b + nr


a = nr-b=(n-1)r + (r-b)


### 3 实现

class ListNode {
int val;
ListNode next;
ListNode(int x) {
val = x;
next = null;
}
}

public class Solution {

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;