linkedListCycle2.java

// 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 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.



/**
 * 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;
        
        if(head == null){
            return null;
        }
        
        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;}
}