var maxDepth = function(root) {

    var fringe = [{ node: root, depth: 1 }];

    var current = fringe.pop();

    var max = 0;

​

    while (current && current.node) {

        var node = current.node;

        // Find all children of this node.

        if (node.left) {

            fringe.push({ node: node.left, depth: current.depth + 1 });

        }

        if (node.right) {

            fringe.push({ node: node.right, depth: current.depth + 1 });

        }

        if (current.depth > max) {

            max = current.depth;

        }

        current = fringe.pop();

    }

​

    return max;

};

​

var insert = function(value, root) {

  if (!root) {

    // Create a new root.

    root = { val: value };

  }

  else {

    var current = root;

    while (current) {

      if (value < current.val) {

        if (!current.left) {

          // Insert left child.

          current.left = { val: value };

          break;

        }

        else {

          current = current.left;

        }

      }

      else if (value > current.val) {

        if (!current.right) {

          // Insert right child.

          current.right = { val: value };

          break;

        }

        else {

          current = current.right;

        }

      }

      else {

        // This value already exists. Ignore it.

        break;

      }

    }

  }

  return root;

}

​

treeToArray = function(head) {

  // Preorder traversal.

  var result = [];

  if (head) {

    result.push(head.val);

    result.push(treeToArray(head.left));

    result.push(treeToArray(head.right));

  }

  return [].concat.apply([], result);

}

​

var root = insert(10);

insert(5, root);

insert(6, root);

insert(3, root);

insert(20, root);

insert(1, root);

​

console.log(maxDepth(root));

​