construct binary tree from preorder and inorder traversal solution

Added time and space complexity comments for clarity.

Author: yhkee0404

construct-binary-tree-from-preorder-and-inorder-traversal/yhkee0404.dart

@@ -0,0 +1,31 @@
+/**
+ * Definition for a binary tree node.
+ * class TreeNode {
+ *   int val;
+ *   TreeNode? left;
+ *   TreeNode? right;
+ *   TreeNode([this.val = 0, this.left, this.right]);
+ * }
+ */
+class Solution {
+  TreeNode? buildTree(List<int> preorder, List<int> inorder) {
+    int i = 1, j = 0;
+    final root = TreeNode(preorder[0]);
+    final stack = [root]; // T(n) = S(n) = O(n)
+    while (i != preorder.length) {
+        var u = stack.last;
+        if (u.val != inorder[j]) {
+            u.left = TreeNode(preorder[i++]);
+            stack.add(u.left!);
+            continue;
+        }
+        while (! stack.isEmpty && stack.last.val == inorder[j]) {
+            u = stack.removeLast();
+            j++;
+        }
+        u.right = TreeNode(preorder[i++]);
+        stack.add(u.right!);
+    }
+    return root;
+  }
+}