添加 二叉树的递归遍历.md Scala版本

problems/二叉树的递归遍历.md

@@ -470,5 +470,56 @@ func postorder(_ root: TreeNode?, res: inout [Int]) {
     res.append(root!.val)
 }
 ```
+Scala: 前序遍历：（144.二叉树的前序遍历）
+```scala
+object Solution {
+  import scala.collection.mutable.ListBuffer
+  def preorderTraversal(root: TreeNode): List[Int] = {
+    val res = ListBuffer[Int]()
+    def traversal(curNode: TreeNode): Unit = {
+      if(curNode == null) return
+      res.append(curNode.value)
+      traversal(curNode.left)
+      traversal(curNode.right)
+    }
+    traversal(root)
+    res.toList
+  }
+}
+```
+中序遍历：（94. 二叉树的中序遍历）
+```scala
+object Solution {  
+  import scala.collection.mutable.ListBuffer
+  def inorderTraversal(root: TreeNode): List[Int] = {
+    val res = ListBuffer[Int]()
+    def traversal(curNode: TreeNode): Unit = {
+      if(curNode == null) return
+      traversal(curNode.left)
+      res.append(curNode.value)
+      traversal(curNode.right)
+    }
+    traversal(root)
+    res.toList
+  }
+}
+```
+后序遍历：（145. 二叉树的后序遍历）
+```scala
+object Solution {
+  import scala.collection.mutable.ListBuffer
+  def postorderTraversal(root: TreeNode): List[Int] = {
+    val res = ListBuffer[Int]()
+    def traversal(curNode: TreeNode): Unit = {
+      if (curNode == null) return
+      traversal(curNode.left)
+      traversal(curNode.right)
+      res.append(curNode.value)
+    }
+    traversal(root)
+    res.toList
+  }
+}
+```
 -----------------------
 <div align="center"><img src=https://code-thinking.cdn.bcebos.com/pics/01二维码一.jpg width=500> </img></div>