142. 环形链表 II

142. 环形链表 II

题解

与 141. 环形链表 不同的点在于本题要求返回环的入口，寻找环的入口是本题难点。

快慢指针

public class Solution {
    public ListNode detectCycle(ListNode head) {
        if (head == null || head.next == null) return null;
        ListNode slow = head;
        ListNode fast = head;
        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
            // 说明有环
            if (fast == slow) {
                ListNode tmp = head;
                // 找到环的第一个
                while (tmp != slow) {
                    tmp = tmp.next;
                    slow = slow.next;
                }
                return tmp;
            }
        }
        return null;
    }
}

哈希表

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