Merge pull request #1351 from wzqwtt/tree02

添加（二叉树的统一迭代法、102.二叉树的层序遍历、107.二叉树的层次遍历II）Scala版本

problems/0102.二叉树的层序遍历.md

@@ -318,6 +318,31 @@ func levelOrder(_ root: TreeNode?) -> [[Int]] {
     return result
 }
 ```
+Scala:
+```scala
+// 102.二叉树的层序遍历
+object Solution {
+  import scala.collection.mutable
+  def levelOrder(root: TreeNode): List[List[Int]] = {
+    val res = mutable.ListBuffer[List[Int]]()
+    if (root == null) return res.toList
+    val queue = mutable.Queue[TreeNode]() // 声明一个队列
+    queue.enqueue(root) // 把根节点加入queue
+    while (!queue.isEmpty) {
+      val tmp = mutable.ListBuffer[Int]()
+      val len = queue.size // 求出len的长度
+      for (i <- 0 until len) { // 从0到当前队列长度的所有节点都加入到结果集
+        val curNode = queue.dequeue()
+        tmp.append(curNode.value)
+        if (curNode.left != null) queue.enqueue(curNode.left)
+        if (curNode.right != null) queue.enqueue(curNode.right)
+      }
+      res.append(tmp.toList)
+    }
+    res.toList
+  }
+}
+```
 
 Rust:
 
@@ -578,6 +603,32 @@ func levelOrderBottom(_ root: TreeNode?) -> [[Int]] {
 }
 ```
 
+
+Scala:
+```scala
+// 107.二叉树的层次遍历II
+object Solution {
+  import scala.collection.mutable
+  def levelOrderBottom(root: TreeNode): List[List[Int]] = {
+    val res = mutable.ListBuffer[List[Int]]()
+    if (root == null) return res.toList
+    val queue = mutable.Queue[TreeNode]()
+    queue.enqueue(root)
+    while (!queue.isEmpty) {
+      val tmp = mutable.ListBuffer[Int]()
+      val len = queue.size
+      for (i <- 0 until len) {
+        val curNode = queue.dequeue()
+        tmp.append(curNode.value)
+        if (curNode.left != null) queue.enqueue(curNode.left)
+        if (curNode.right != null) queue.enqueue(curNode.right)
+      }
+      res.append(tmp.toList)
+    }
+    // 最后翻转一下
+    res.reverse.toList
+  }
+
 Rust:
 
 ```rust

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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