code: function initChildren(node, isAsc) {
      var children = node.children || [];
  
      node.children = sort(children, isAsc);
  
      // Init children recursively
      if (children.length) {
          zrUtil.each(node.children, function (child) {
              initChildren(child, isAsc);
          });
      }
  }
docs: Init node children by order and update visual
  
  @param {TreeNode} node  root node
  @param {boolean}  isAsc if is in ascendant order