LeetCode-142-Circular Linked List II

Ring List II

Topic description: Given a linked list, return the first node where the linked list begins to enter the ring. Returns null if the linked list has no cycle.

To represent a cycle in a given linked list, we use the integer pos to denote the position in the linked list where the tail of the linked list is connected (indexes start at 0). If pos is -1, there are no cycles in the list. Note that pos is only used to identify the ring case and is not passed as an argument to the function.

Description: Modification of the given linked list is not allowed.

Advanced:

Can you solve this problem in O(1) space?

For example descriptions, please refer to the official website of LeetCode.

Source: LeetCode
Link: https://leetcode-cn.com/problems/linked-list-cycle-ii/
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Solution 1: Hash table

HashSet determines whether the node is repeated.

Solution 2: Double pointer method

Using fast and slow nodes, if there is a cycle, these two nodes must meet.

import java.util.HashSet;
import java.util.Set;

public class LeetCode_142 {
    /**
     * 哈希
     *
     * @param head
     * @return
     */
    public static ListNode detectCycle(ListNode head) {
        if (head == null) {
            return null;
        }
        Set<ListNode> nodes = new HashSet<>();
        nodes.add(head);
        while (head.next != null) {
            // 如果next节点在哈希表中出现过，说明成环了， 而且next节点肯定是入环点，直接返回
            if (nodes.contains(head.next)) {
                return head.next;
            }
            nodes.add(head.next);
            head = head.next;
        }
        return null;
    }

    /**
     * 双指针法
     *
     * @param head
     * @return
     */
    public static ListNode detectCycle2(ListNode head) {
        if (head == null) {
            return null;
        }
        ListNode slow = head, fast = head;
        while (fast != null) {
            slow = slow.next;
            if (fast.next != null) {
                fast = fast.next.next;
            } else {
                return null;
            }
            if (fast == slow) {
                ListNode ptr = head;
                while (ptr != slow) {
                    ptr = ptr.next;
                    slow = slow.next;
                }
                return ptr;
            }
        }
        return null;
    }

    public static void main(String[] args) {
        ListNode head = new ListNode(3);
        head.next = new ListNode(2);
        head.next.next = new ListNode(0);
        head.next.next.next = new ListNode(-4);
        head.next.next.next.next = head.next;

        System.out.println(detectCycle(head) == null ? -1 : detectCycle(head).val);
        System.out.println(detectCycle2(head) == null ? -1 : detectCycle2(head).val);
    }
}

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