添加 二叉树的迭代遍历.md Scala版本

problems/二叉树的迭代遍历.md

@@ -568,6 +568,71 @@ func inorderTraversal(_ root: TreeNode?) -> [Int] {
     return result
 }
 ```
+Scala:
+```scala
+// 前序遍历（迭代法）
+object Solution {
+  import scala.collection.mutable
+  def preorderTraversal(root: TreeNode): List[Int] = {
+    val res = mutable.ListBuffer[Int]()
+    if (root == null) return res.toList
+    // 声明一个栈，泛型为TreeNode
+    val stack = mutable.Stack[TreeNode]()
+    stack.push(root) // 先把根节点压入栈
+    while (!stack.isEmpty) {
+      var curNode = stack.pop()
+      res.append(curNode.value) // 先把这个值压入栈
+      // 如果当前节点的左右节点不为空，则入栈，先放右节点，再放左节点
+      if (curNode.right != null) stack.push(curNode.right)
+      if (curNode.left != null) stack.push(curNode.left)
+    }
+    res.toList
+  }
+}
 
+// 中序遍历（迭代法）
+object Solution {
+  import scala.collection.mutable
+  def inorderTraversal(root: TreeNode): List[Int] = {
+    val res = mutable.ArrayBuffer[Int]()
+    if (root == null) return res.toList
+    val stack = mutable.Stack[TreeNode]()
+    var curNode = root
+    // 将左节点都入栈，当遍历到最左（到空）的时候，再弹出栈顶元素，加入res
+    // 再把栈顶元素的右节点加进来，继续下一轮遍历
+    while (curNode != null || !stack.isEmpty) {
+      if (curNode != null) {
+        stack.push(curNode)
+        curNode = curNode.left
+      } else {
+        curNode = stack.pop()
+        res.append(curNode.value)
+        curNode = curNode.right
+      }
+    }
+    res.toList
+  }
+}
+
+// 后序遍历（迭代法）
+object Solution {
+  import scala.collection.mutable
+  def postorderTraversal(root: TreeNode): List[Int] = {
+    val res = mutable.ListBuffer[Int]()
+    if (root == null) return res.toList
+    val stack = mutable.Stack[TreeNode]()
+    stack.push(root)
+    while (!stack.isEmpty) {
+      val curNode = stack.pop()
+      res.append(curNode.value)
+      // 这次左节点先入栈，右节点再入栈
+      if(curNode.left != null) stack.push(curNode.left)
+      if(curNode.right != null) stack.push(curNode.right)
+    }
+    // 最后需要翻转List
+    res.reverse.toList
+  }
+}
+```
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