The unique identifier of node's parent node.
1. zTree v3.x using 'parentTId' replaced the original 'parentNode' attribute, and increased getParentNode () method, in order to avoid the original 'parentNode' cause the clone () method infinite loop.
2. Do not initialize or modify it, it is created by the zTree.
String object of node's parent node's tId. please see API about 'treeNode.tId'
If treeNode is root node, parentTId is null.
var treeObj = $.fn.zTree.getZTreeObj("tree");
var sNodes = treeObj.getSelectedNodes();
if (sNodes.length > 0) {
var parentTId = sNodes[0].parentTId;
}