添加 0098.验证二叉搜索树.md Scala版本

problems/0098.验证二叉搜索树.md

@@ -589,7 +589,50 @@ function isValidBST(root: TreeNode | null): boolean {
 };
 ```
 
+## Scala
 
+辅助数组解决:
+```scala
+object Solution {
+  import scala.collection.mutable
+  def isValidBST(root: TreeNode): Boolean = {
+    var arr = new mutable.ArrayBuffer[Int]()
+    // 递归中序遍历二叉树，将节点添加到arr
+    def traversal(node: TreeNode): Unit = {
+      if (node == null) return
+      traversal(node.left)
+      arr.append(node.value)
+      traversal(node.right)
+    }
+    traversal(root)
+    // 这个数组如果是升序就代表是二叉搜索树
+    for (i <- 1 until arr.size) {
+      if (arr(i) <= arr(i - 1)) return false
+    }
+    true  
+  }
+}
+```
+
+递归中解决:
+```scala
+object Solution {
+  def isValidBST(root: TreeNode): Boolean = {
+    var flag = true
+    var preValue:Long = Long.MinValue // 这里要使用Long类型
+
+    def traversal(node: TreeNode): Unit = {
+      if (node == null || flag == false) return
+      traversal(node.left)
+      if (node.value > preValue) preValue = node.value
+      else flag = false
+      traversal(node.right)
+    }
+    traversal(root)
+    flag  
+  }
+}
+```
 
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