Preorder Traversal

Visit the root before each subtree, producing root-left-right order.

Algorithm

The canonical tree is 4(2(1,3),6(5,7)), so this Dart DSA implementation can be compared directly with the rest of the DSA track.

Basic Implementation

basic.dart

class Node {
  Node(this.value, [this.left, this.right]);
  final int value;
  Node? left;
  Node? right;
}
String render(Node? node) {
  if (node == null) return "_";
  if (node.left == null && node.right == null) return node.value.toString();
  return "${node.value}(${render(node.left)},${render(node.right)})";
}
Node sampleTree() => Node(4, Node(2, Node(1), Node(3)), Node(6, Node(5), Node(7)));
String listString(List<int> values) => "[${values.join(", ")}]";
void preorder(Node? node, List<int> output) { if (node == null) return; output.add(node.value); preorder(node.left, output); preorder(node.right, output); }
void main() { final output = <int>[]; preorder(sampleTree(), output); print(listString(output)); }

Complexity

Time: O(n)
Space: O(h) recursion stack

Implementation notes

Render tree structure explicitly instead of printing node objects.
The replay highlights the node, traversal state, queue, path, or search cursor that changes at each step.

preorder Preorder records the current node before visiting left and right subtrees.