kt: add «889. Construct Binary Tree from Preorder and Postorder Traversal»

URL:	https://leetcode.com/problems/construct-binary-tree-from-preorder-and-postorder-traversal/
Signed-off-by: Matej Focko <me@mfocko.xyz>

kt/construct-binary-tree-from-preorder-and-postorder-traversal.kt

@@ -0,0 +1,27 @@
+class Solution {
+    private data class Reconstruct(val preorder: IntArray, val postorder: IntArray) {
+        private var pre: Int = 0
+        private var post: Int = 0
+
+        fun construct(): TreeNode? {
+            val node = TreeNode(preorder[pre])
+            ++pre
+
+            if (node.`val` != postorder[post]) {
+                node.left = construct()
+            }
+
+            if (node.`val` != postorder[post]) {
+                node.right = construct()
+            }
+
+            ++post
+            return node
+        }
+    }
+
+    fun constructFromPrePost(
+        preorder: IntArray,
+        postorder: IntArray,
+    ): TreeNode? = Reconstruct(preorder, postorder).construct()
+}