Merge pull request youngyangyang04#1348 from wzqwtt/tree01

添加（二叉树理论基础、二叉树的递归遍历、二叉树的迭代遍历）Scala版本

Author: youngyangyang04

problems/二叉树理论基础.md

@@ -258,6 +258,13 @@ class TreeNode<T> {
     }
 }
 ```
-
+Scala:
+```scala
+class TreeNode(_value: Int = 0, _left: TreeNode = null, _right: TreeNode = null) {
+  var value: Int = _value
+  var left: TreeNode = _left
+  var right: TreeNode = _right
+}
+```
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 <div align="center"><img src=https://code-thinking.cdn.bcebos.com/pics/01二维码一.jpg width=500> </img></div>

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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 <div align="center"><img src=https://code-thinking.cdn.bcebos.com/pics/01二维码一.jpg width=500> </img></div>

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
+  }
+}
+```
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 <div align="center"><img src=https://code-thinking.cdn.bcebos.com/pics/01二维码一.jpg width=500> </img></div>