141. Linked List Cycle

Given the head of a linked list, return the node where the cycle begins. If there is no cycle, return null.

There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail's next pointer is connected to (0-indexed). It is -1 if there is no cycle. Note that pos is not passed as a parameter.

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 = [1], pos = -1
Output: no cycle
Explanation: There is no cycle in the linked list.

Constraints:

• The number of the nodes in the list is in the range [0, 10^4].
• -10^5 <= Node.val <= 10^5
• pos is -1 or a valid index in the linked-list.

Follow-up: Can you solve it using O(1) (i.e. constant) memory?

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def detectCycle(self, head: Optional[ListNode]) -> Optional[ListNode]:
        slow = fast = head

        while fast != None and fast.next != None:
            slow = slow.next
            fast = fast.next.next
            if slow == fast:
                break

        if fast == None or fast.next == None:
            return None

        slow = head
        while fast != slow:
            slow = slow.next
            fast = fast.next

        return slow

/**
 * 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) {
        ListNode slow = head;
        ListNode fast = head;

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

        if (fast == null || fast.next == null) {
            return null;
        }

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

        return head;
    }
}

//   Definition for singly-linked list.
class ListNode {
  val: number;
  next: ListNode | null;
  constructor(val?: number, next?: ListNode | null) {
    this.val = val === undefined ? 0 : val;
    this.next = next === undefined ? null : next;
  }
}

function detectCycle(head: ListNode | null): ListNode | null {
  let slow = head;
  let fast = head;

  while (fast && fast.next) {
    fast = fast.next.next;
    slow = slow.next;
    if (fast === slow) break;
  }

  if (fast === null || fast.next === null) {
    return null;
  }

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

  return head;
}