链表是否有环，求交点

链表是否有环

private static class ListNode {
        private int val;
        private ListNode next;

        public ListNode() {
        }

        public ListNode(int val) {
            this.val = val;
        }

        @Override
        public String toString() {
            return val +"";
        }
    }

    public static ListNode meetingNode(ListNode head) {

        ListNode fast = head;
        ListNode slow = head;

        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
            if (fast == slow) {
                break;
            }
        }

        // 链表中没有环
        if (fast == null || fast.next == null) {
            return null;
        }

        // fast重新指向第一个结点
        fast = head;

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

        return fast;
    }