# Mathematica, 38 bytes #

    FromDigits@*Join@@Range[Range@#,1,-1]&

I stole `Range[Range@#,1,-1]` from [Martin Ender's (still shorter) answer][1] to save three bytes (dammit, I gotta start remembering that some functions are Listable). `Join@@` flattens them into a single list, which `FromDigits` converts into a single integer.


  [1]: http://codegolf.stackexchange.com/a/103706/56178