https://leetcode.com/problems/linked-list-cycle-ii/
Approach 1: Hash Table1
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26/**
* 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;
}
Set<ListNode> set = new HashSet<>();
while (!set.contains(head)) {
set.add(head);
head = head.next;
if (head == null)
return null;
}
return head;
}
}
Approach 2: Floyd’s Tortoise and Hare
Explanation1
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18public class Solution {
public ListNode detectCycle(ListNode head) {
ListNode fast = head, slow = head;
while (fast != null && fast.next != null) {
slow = slow.next;
fast = fast.next.next;
if (slow == fast) {
slow = head;
while (slow != fast) {
slow = slow.next;
fast = fast.next;
}
return slow;
}
}
return null;
}
}