Get the treeNode's index in its parent node's children.
Do not initialize or modify it, it is created by the zTree.
the treeNode's index in its parent node's children. ( start at 0 )
var treeObj = $.fn.zTree.getZTreeObj("tree");
var sNodes = treeObj.getSelectedNodes();
if (sNodes.length > 0) {
var node = sNodes[0].getIndex();
}