Comment on page

## 题目描述

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 = , pos = -1
Output: no cycle
Explanation: There is no cycle in the linked list.
Follow-up:
Can you solve it without using extra space?

## 代码

### Approach #1 Hash Table

/**
* class ListNode {
* int val;
* ListNode next;
* ListNode(int x) {
* val = x;
* next = null;
* }
* }
*/
public class Solution {
Set<ListNode> visited = new HashSet<ListNode>();
while (node != null) {
if (visited.contains(node)) {
return node;
}
node = node.next;
}
return null;
}
}

### Approach 2: Floyd's Tortoise and Hare

public class Solution {
if (head == null) return null;
if (intersect == null) return null;
ListNode ptr2 = intersect;
while (ptr1 != ptr2) {
ptr1 = ptr1.next;
ptr2 = ptr2.next;
}
return ptr1;
}