kt: add «1028. Recover a Tree From Preorder Traversal»

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

kt/recover-a-tree-from-preorder-traversal.kt

@@ -0,0 +1,37 @@
+class Solution {
+    fun recoverFromPreorder(traversal: String): TreeNode? {
+        val levels = mutableListOf<TreeNode>()
+
+        var index = 0
+        while (index < traversal.length) {
+            var depth = 0
+            while (index < traversal.length && traversal[index] == '-') {
+                ++depth
+                ++index
+            }
+
+            var value = 0
+            while (index < traversal.length && traversal[index].isDigit()) {
+                value = 10 * value + (traversal[index] - '0')
+                ++index
+            }
+
+            val node = TreeNode(value)
+            if (depth < levels.size) {
+                levels[depth] = node
+            } else {
+                levels.add(node)
+            }
+
+            if (depth > 0) {
+                val parent = levels[depth - 1]
+                when {
+                    parent.left == null -> parent.left = node
+                    else -> parent.right = node
+                }
+            }
+        }
+
+        return levels[0]
+    }
+}