Merge pull request #1352 from wzqwtt/tree03

添加（199.二叉树的右视图、637.二叉树的层平均值、429.N叉树的层序遍历）Scala版本

problems/0102.二叉树的层序遍历.md

@@ -880,6 +880,31 @@ func rightSideView(_ root: TreeNode?) -> [Int] {
 }
 ```
 
+Scala:
+```scala
+// 199.二叉树的右视图
+object Solution {
+  import scala.collection.mutable
+  def rightSideView(root: TreeNode): List[Int] = {
+    val res = mutable.ListBuffer[Int]()
+    if (root == null) return res.toList
+    val queue = mutable.Queue[TreeNode]()
+    queue.enqueue(root)
+    while (!queue.isEmpty) {
+      val len = queue.size
+      var curNode: TreeNode = null
+      for (i <- 0 until len) {
+        curNode = queue.dequeue()
+        if (curNode.left != null) queue.enqueue(curNode.left)
+        if (curNode.right != null) queue.enqueue(curNode.right)
+      }
+      res.append(curNode.value)   // 把最后一个节点的值加入解集
+    }
+    res.toList  // 最后需要把res转换为List，return关键字可以省略
+  }
+}
+```
+
 # 637.二叉树的层平均值
 
 [力扣题目链接](https://leetcode-cn.com/problems/average-of-levels-in-binary-tree/)
@@ -1111,6 +1136,30 @@ func averageOfLevels(_ root: TreeNode?) -> [Double] {
     return result
 }
 ```
+Scala:
+```scala
+// 637.二叉树的层平均值
+object Solution {
+  import scala.collection.mutable
+  def averageOfLevels(root: TreeNode): Array[Double] = {
+    val res = mutable.ArrayBuffer[Double]()
+    val queue = mutable.Queue[TreeNode]()
+    queue.enqueue(root)
+    while (!queue.isEmpty) {
+      var sum = 0.0
+      var len = queue.size
+      for (i <- 0 until len) {
+        var curNode = queue.dequeue()
+        sum += curNode.value // 累加该层的值
+        if (curNode.left != null) queue.enqueue(curNode.left)
+        if (curNode.right != null) queue.enqueue(curNode.right)
+      }
+      res.append(sum / len) // 平均值即为sum/len
+    }
+    res.toArray // 最后需要转换为Array，return关键字可以省略
+  }
+}
+```
 
 # 429.N叉树的层序遍历
 
@@ -1355,6 +1404,34 @@ func levelOrder(_ root: Node?) -> [[Int]] {
 }
 ```
 
+Scala:
+```scala
+// 429.N叉树的层序遍历
+object Solution {
+  import scala.collection.mutable
+  def levelOrder(root: Node): List[List[Int]] = {
+    val res = mutable.ListBuffer[List[Int]]()
+    if (root == null) return res.toList
+    val queue = mutable.Queue[Node]()
+    queue.enqueue(root) // 根节点入队
+    while (!queue.isEmpty) {
+      val tmp = mutable.ListBuffer[Int]() // 存储每层节点
+      val len = queue.size
+      for (i <- 0 until len) {
+        val curNode = queue.dequeue()
+        tmp.append(curNode.value) // 将该节点的值加入tmp
+        // 循环遍历该节点的子节点，加入队列
+        for (child <- curNode.children) {
+          queue.enqueue(child)
+        }
+      }
+      res.append(tmp.toList) // 将该层的节点放到结果集
+    }
+    res.toList
+  }
+}
+```
+
 # 515.在每个树行中找最大值
 
 [力扣题目链接](https://leetcode-cn.com/problems/find-largest-value-in-each-tree-row/)