problems: add binary tree level-order traversal

problems/binary-tree-level-order-traversal.kt

@@ -0,0 +1,27 @@
+/**
+ * Example:
+ * var ti = TreeNode(5)
+ * var v = ti.`val`
+ * Definition for a binary tree node.
+ * class TreeNode(var `val`: Int) {
+ *     var left: TreeNode? = null
+ *     var right: TreeNode? = null
+ * }
+ */
+class Solution {
+    private fun levelOrder(root: TreeNode?, levels: MutableList<MutableList<Int>>, level: Int): MutableList<MutableList<Int>> {
+        if (root == null) {
+            return levels
+        }
+        
+        if (level + 1 > levels.size) {
+            levels.add(mutableListOf<Int>())
+        }
+        
+        levels[level].add(root.`val`)
+        levelOrder(root.left, levels, level + 1)
+        return levelOrder(root.right, levels, level + 1)
+    }
+    
+    fun levelOrder(root: TreeNode?): List<List<Int>> = levelOrder(root, mutableListOf<MutableList<Int>>(), 0)
+}