142. Linked List Cycle II

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https://leetcode.com/problems/linked-list-cycle-ii/

Approach 1: Hash Table

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/**
 * 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
Explanation
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public 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;
    }
}

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