problems(kt): add „606. Construct String from Binary Tree“

Signed-off-by: Matej Focko <mfocko@redhat.com>

problems/construct-string-from-binary-tree.kt

@@ -0,0 +1,30 @@
+/**
+ * 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 {
+    fun tree2str(root: TreeNode?): String {
+        if (root == null) {
+            return ""
+        }
+
+        val value = root.`val`.toString()
+        val left = "(${tree2str(root.left)})"
+        val right = "(${tree2str(root.right)})"
+
+
+        if (left == "()" && right == "()") {
+            return value
+        } else if (right == "()") {
+            return "$value$left"
+        }
+
+        return "$value$left$right"
+    }
+}