One way is to create a recursive function that traverses the tree.
I assume you only want to save the data in the leaf nodes.
Qt Code: 
  1. void SomeClass::traverseTree(QTreeWidgetItem root) 
  2. {
  3.   if( root && root->childCount()) 
  4.   {
  5.     int childCount = root->childCount();
  6.     for( int i = 0; i != childCount(); i++ )
  7.       traverseTree( root->child(i);
  8.     //Assuming you have any extra data in items that are not terminal nodes, 
  9.     //then you can save it here.
  10.   }
  11.   else if( !root->childCount() ) //this means it is a leaf
  12.   {
  13.    //save whatever data you have in your item in the file.
  14.   }
  15. }
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This should pretty much work.

Regards