merge   4 +   6 =  10
merge  10 +  10 =  20
merge  15 +  20 =  35
merge  25 +  35 =  60
merge  40 +  60 = 100

 sym  freq  code      bits
   W    40      0        40
   D    25     10        50
   P    15    110        45
   F    10   1110        40
   S     6  11111        30
   I     4  11110        20

Huffman total          = 225 bits
sum of internal nodes  = 225 bits (same number)
fixed-length 3 bits    = 300 bits
saving                 = 25 %
