142. Linked List Cycle II

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

Note: Do not modify the linked list.

still two pointers, but DON'T give any pointer a head-start, cause in reality there is no such thing. in the while loop, run the pointers first, then check

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

        while(fast != null){
            slow = slow.next;
            fast = fast.next;
            if(fast != null) fast = fast.next;
            if(fast == slow){
                fast = head;
                while(slow != fast){
                    fast = fast.next;
                    slow = slow.next;
                }
                return slow;
            }


        }

        return null;



    }
}