Leetcode 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.

Follow up:
Can you solve it without using extra space?

 

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode DetectCycle(ListNode head) {
        if (head == null) return null;
        
        ListNode fast = head, slow = head, entry = head;
        
        while (fast.next != null && fast.next.next != null)
        {
            fast = fast.next.next;
            slow = slow.next;
            
            // found a cycle
            if (slow == fast)
            {
                while (slow != entry)
                {
                    slow = slow.next;
                    entry = entry.next;
                }
                
                return entry;
            }
        }
        
        return null;
    }
}