How do I add odd/even class only for table rows which haven´t had odd/even class yet? (for some tables I use php cycle function)

How do I have to change this code?

  

   
$("table tbody tr td")
   
 .parent("tr:nth-child(odd)")
   
 .addClass("odd")
   
 .end()
   
 .parent("tr:nth-child(even)")
   
 .addClass("even");