My solution to http://cslibrary.stanford.edu/109/TreeListRecursion.html. I think mine is more efficient than Nick's.

gistfile1.dart

// This is a Dart solution to:
//
// Nick Parlante's "Great Tree-List Recursion Problem"
// http://cslibrary.stanford.edu/109/TreeListRecursion.html
//
// Before you read the code here, try to solve it youself first!

main() {
  var tree = makeTree(100);
  printTree(tree);
  var list = treeToList(tree);
  printList(list);
}

// Here's the solution:

class Node {
  final int data;
  Node left;
  Node right;

  Node(this.data);
}

/// Convert [tree] to a linked list and return the head node.
Node treeToList(Node tree) {
  var previous;
  var last;

  // Traverse the tree in order, and replace every left pointer to its previous
  // one instead of its child. (We can't do the right pointers here because
  // we haven't traversed those yet.)
  inOrder(tree, (node) {
    last = node;
    node.left = previous;
    previous = node;
  });

  // Now walk the list along the previous chain and wire up the next pointers.
  while (previous.left != null) {
    previous.left.right = previous;
    previous = previous.left;
  }

  // Make the list circular.
  previous.left = last;
  last.right = previous;

  // Since we walked back to the beginning, this is now the head.
  return previous;
}

/// Invoke [callback] on each node in [tree], in order.
void inOrder(Node tree, callback) {
  if (tree.left != null) inOrder(tree.left, callback);
  callback(tree);
  if (tree.right != null) inOrder(tree.right, callback);
}

// Scaffolding:

Node makeTree(int numNodes) {
  var nums = new Iterable.generate(numNodes, (i) => i).toList();
  nums.shuffle();

  var root = new Node(nums[0]);
  for (var n in nums.skip(1)) {
    insertInTree(root, n);
  }
  return root;
}

void insertInTree(Node tree, int n) {
  if (n < tree.data) {
    if (tree.left == null) {
      tree.left = new Node(n);
    } else {
      insertInTree(tree.left, n);
    }
  } else {
    if (tree.right == null) {
      tree.right = new Node(n);
    } else {
      insertInTree(tree.right, n);
    }
  }
}

void printTree(Node tree, [String indent = ""]) {
  if (tree.left != null) printTree(tree.left, indent + "  ");
  print("$indent${tree.data}");
  if (tree.right != null) printTree(tree.right, indent + "  ");
}

void printList(Node head) {
  var node = head;
  do {
    print("${node.left.data} <- ${node.data} -> ${node.right.data}");
    node = node.right;
  } while (node != head);
}