修改（0142.环形链表II.md）：优化typescript版本代码

Author: xiaofei-2020

problems/0142.环形链表II.md

@@ -301,13 +301,13 @@ function detectCycle(head: ListNode | null): ListNode | null {
     let slowNode: ListNode | null = head,
         fastNode: ListNode | null = head;
     while (fastNode !== null && fastNode.next !== null) {
-        slowNode = (slowNode as ListNode).next;
+        slowNode = slowNode!.next;
         fastNode = fastNode.next.next;
         if (slowNode === fastNode) {
             slowNode = head;
             while (slowNode !== fastNode) {
-                slowNode = (slowNode as ListNode).next;
-                fastNode = (fastNode as ListNode).next;
+                slowNode = slowNode!.next;
+                fastNode = fastNode!.next;
             }
             return slowNode;
         }