title: d3.js Collapsible Tree node should drop on first parent or child (+1 or -1)
body: <p>Working on **D3.js Drag and Drop, Zoomable, Panning, Collapsible Tree, I want node element to be drop on exact parent or child node i.e (+1 or -1) i have tried various conditions but not fulfilling.</p>
  
  <p>Currently it drag-drop on any node.</p>
  
  <p>here is an example <code>[http://jsfiddle.net/HarishBoke/8wYfu/][1]</code></p>
  
  <p>Thanks in advance!!!</p>