function createTestTree2() {
  var nodes = new Array(7);
  for (var i = 0; i < nodes.length; i++) {
    nodes[i] = new TreeNode_(i);
  }

  // Creating a tree that has the following structure.
  //           n0
  //           |
  //           n1
  //          /  \
  //        n2    n6
  //       /  \
  //      n3  n4
  //            \
  //            n5

  nodes[0].addChild(nodes[1]);

  nodes[1].addChild(nodes[2]);
  nodes[1].addChild(nodes[6]);

  nodes[2].addChild(nodes[3]);
  nodes[2].addChild(nodes[4]);

  nodes[4].addChild(nodes[5]);

  return nodes;
}
Creates a different tree to be used in various tests.
@return {!Array<!TreeNode_>} An array holding all the nodes in
    the tree in pre-order traversal order.