[LeetCode] Problem 142 - Linked List Cycle II

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

No.1

Input: head = [3,2,0,-4], pos = 1

Output: true

Explanation: There is a cycle in the linked list, where tail connects to the second node.

No.2

Input: head = [1,2], pos = 0

Output: true

Explanation: There is a cycle in the linked list, where tail connects to the first node.

No.3

Input: head = [1], pos = -1

Output: false

Explanation: There is no cycle in the linked list.

Follow up

Can you solve it without using extra space?

Code

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

public ListNode detectCycle(ListNode head) {
    ListNode result = hasCycle(head);

    if (result == null)
        return null;

    while (head != result) {
        head = head.next;
        result = result.next;
    }

    return result;
}

private ListNode hasCycle(ListNode head) {
    if (head == null)
        return null;

    ListNode fast = head;
    ListNode slow = head;

    while (fast.next != null && fast.next.next != null) {
        fast = fast.next.next;
        slow = slow.next;

        if (fast == slow)
            return slow;
    }

    return null;
}