Algorithm Pre-processing (InitStringSet)
Input: InitStringSet: a initial string set.
Output: Σ :common alphabet over InitStringSet
     StringSet: the result of pre-processed InitStringSet
(1)  for each string in InitStringSet
(2)  tempSet =
(3)  for each character   [j] in
(4)   if ( )
(5)    tempSet = tempSet     [j]}
(6)    if ( [j] map)
(7)     map ( [j]) = map ( [j]) + 1
(8)    else
(9)     map ( [j]) = 1
(10)  endfor
(11) endfor
(12)
(13) for each character  char[i] in map
(14) if (map(char[i]) == size (InitStringSet))
(15)   Σ = Σ    {char[i]}
(16) endfor
(17) StringSet = InitStringSet
(18) for each string in StringSet
(19)  for each character   [j] in
(20)    if ( [j] Σ)
(21)     delete ( [j])
(22)  endfor
(23) endfor
(24) return Σ and StringSet
Algorithm 1: The pseudocode of the preprocessing algorithm.