添加 链表理论基础.md Scala版本

problems/链表理论基础.md

@@ -210,6 +210,13 @@ type ListNode struct {
 }
 ```
 
+Scala:
+```scala
+class ListNode(_x: Int = 0, _next: ListNode = null) {
+  var next: ListNode = _next
+  var x: Int = _x
+}
+```
 
 
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