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public class Solution {
    public ListNode detectCycle(ListNode head) {
        if (head == null) {
            return null;
        }

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