How to use same tree after use it in same function in Haskell Recursive Function

data HuffmanTree 
   = Empty 
   | Intermediate Integer HuffmanTree HuffmanTree 
   | Leaf Char Integer
   deriving (Show,Eq)

decode :: HuffmanTree -> [Char] -> [Char]
decode (Intermediate n x y) (z:zs) = 
   if ..... (some checks for x,y,z)..... 
   then [getChar x] ++ (decode (Intermediate n x y) zs)

in this function I m moving on a tree. At "then" part I take deepest Leaf's x and after "++" I should give whole input tree again.The problem is : After ++ part still my function accepts my current tree (deepest leaf's parent). How to fix it ?