Linked List Cycle II

Given a linked list, return the node where the cycle begins.

If there is no cycle, return null.

For example:

Given -21->10->4->5, tail connects to node index 1，return 10

/**
 * Definition for ListNode.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int val) {
 *         this.val = val;
 *         this.next = null;
 *     }
 * }
 */ 

/*
time:O(n), space:O(1)
*/

public class Solution {
    public ListNode detectCycle(ListNode head) { 
        if (head == null) {
            return head;
        }
        ListNode slow = head;
        ListNode fast = head.next;
        while (slow != fast) {
            if (fast == null || fast.next == null) {
                return null;
            }
            slow = slow.next;
            fast = fast.next.next;
        }
        while (head != slow.next) {
            head = head.next;
            slow = slow.next;
        }
        return head;
    }
}