The following procedure takes as its argument a list of symbol-frequency pairs (where no symbol appears in more than one pair) and generates a Huffman encoding tree according to the Huffman algorithm.
(define (generate-huffman-tree pairs) (successive-merge (make-leaf-set pairs)))
Make-leaf-set is the procedure given above that transforms the list of pairs into an ordered set of leaves.
Successive-merge is the procedure you must write, using
make-code-tree to successively merge the smallest-weight elements of the set until there
is only one element left, which is the desired Huffman tree. (This
procedure is slightly tricky, but not really complicated. If you find
yourself designing a complex procedure, then you are almost certainly
doing something wrong. You can take significant advantage of the fact
that we are using an ordered set representation.)