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

Note:Do not modify the linked list.

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

tag: linkedlist

/**
 * 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) return head;

        ListNode slow = head, fast = head;
        boolean found = false;
        while (fast != null && fast.next != null){
            slow = slow.next;
            fast = fast.next.next;

            if (slow == fast){
                found = true;
                break;
            }
        }
        if (!found) return null;

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

        return slow;
    }
}