function createTestTree1() {
  var nodes = new Array(11);
  for (var i = 0; i < nodes.length; i++) {
    nodes[i] = new TreeNode_(i);
  }

  // Creating a tree that has the following structure.
  //            n0
  //          / | \
  //         /  |  \
  //        /   |  n10
  //       /    |
  //      n1    n5
  //    / | \   |
  //  n2 n3 n4  n6
  //            |
  //            n7
  //           /  \
  //         n8   n9

  nodes[1].addChild(nodes[2]);
  nodes[1].addChild(nodes[3]);
  nodes[1].addChild(nodes[4]);

  nodes[5].addChild(nodes[6]);
  nodes[6].addChild(nodes[7]);

  nodes[7].addChild(nodes[8]);
  nodes[7].addChild(nodes[9]);

  nodes[0].addChild(nodes[1]);
  nodes[0].addChild(nodes[5]);
  nodes[0].addChild(nodes[10]);

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