leetcode-142. 环形链表 II

原始思路

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public class Solution {
    Set<ListNode> set = new HashSet<>();
    public ListNode detectCycle(ListNode head) {
        if (head == null) {
            return null;
        }
        if (set.contains(head)){
            return head;
        }
        set.add(head);
        if (head.next!=null){
            return detectCycle(head.next);
        }
        return null;
    }
}