LeetCode142-环形链表II

LeetCode142-环形链表II

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
public ListNode detectCycle(ListNode head) {
        ListNode fast = head, snow = head;
        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            snow = snow.next;
            //两指针相遇了
            if (snow == fast) break;
        }
        if (fast == null || fast.next == null) {
            // fast 遇到空指针说明没有环
            return null;
        }
        //否则有环
        //将慢指针从头结点重新开始走
        snow = head;
        while (snow != fast) {
            snow = snow.next;
            fast = fast.next;
        }
        return snow;
    }