添加 二叉树的统一迭代法.md Scala版本

problems/二叉树的统一迭代法.md

@@ -591,6 +591,80 @@ function postorderTraversal(root: TreeNode | null): number[] {
     return res;
 };
 ```
+Scala:
+```scala
+// 前序遍历
+object Solution {
+  import scala.collection.mutable
+  def preorderTraversal(root: TreeNode): List[Int] = {
+    val res = mutable.ListBuffer[Int]()
+    val stack = mutable.Stack[TreeNode]()
+    if (root != null) stack.push(root)
+    while (!stack.isEmpty) {
+      var curNode = stack.top
+      if (curNode != null) {
+        stack.pop()
+        if (curNode.right != null) stack.push(curNode.right)
+        if (curNode.left != null) stack.push(curNode.left)
+        stack.push(curNode)
+        stack.push(null)
+      } else {
+        stack.pop()
+        res.append(stack.pop().value)
+      }
+    }
+    res.toList
+  }
+}
 
+// 中序遍历
+object Solution {  
+  import scala.collection.mutable
+  def inorderTraversal(root: TreeNode): List[Int] = {
+    val res = mutable.ListBuffer[Int]()
+    val stack = mutable.Stack[TreeNode]()
+    if (root != null) stack.push(root)
+    while (!stack.isEmpty) {
+      var curNode = stack.top
+      if (curNode != null) {
+        stack.pop()
+        if (curNode.right != null) stack.push(curNode.right)
+        stack.push(curNode)
+        stack.push(null)
+        if (curNode.left != null) stack.push(curNode.left)
+      } else {
+        // 等于空的时候好办，弹出这个元素
+        stack.pop()
+        res.append(stack.pop().value)
+      }
+    }
+    res.toList
+  }
+}
+
+// 后序遍历
+object Solution {
+  import scala.collection.mutable
+  def postorderTraversal(root: TreeNode): List[Int] = {
+    val res = mutable.ListBuffer[Int]()
+    val stack = mutable.Stack[TreeNode]()
+    if (root != null) stack.push(root)
+    while (!stack.isEmpty) {
+      var curNode = stack.top
+      if (curNode != null) {
+        stack.pop()
+        stack.push(curNode)
+        stack.push(null)
+        if (curNode.right != null) stack.push(curNode.right)
+        if (curNode.left != null) stack.push(curNode.left)
+      } else {
+        stack.pop()
+        res.append(stack.pop().value)
+      }
+    }
+    res.toList
+  }
+}
+```
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