SML—括号最大匹配距离问题（一）



暴力求解算法

fun parenMatch (parens : paren seq) : bool =
  let
   fun pm ((NONE, _) | (SOME 0, CPAREN)) = NONE
      | pm (SOME c, CPAREN) = SOME (c-1)
      | pm (SOME c, OPAREN) = SOME (c+1)
  in
      iter pm (SOME 0) parens = (SOME 0)
  end

fun parenDist (parens : paren seq) : int option =
 let
   val n = Seq.length parens
   fun myBF(i,j,max) = case (parenMatch (Seq.subseq parens (i,j))) of
       false => if i = n-1 then NONE
       else if j<n-i then myBF(i,j+1,max)
       else myBF(i+1,1,max)
       |true => if j=n-i then Option210.intMax(max,SOME(j-2))
       else myBF(i+j,1,Option210.intMax(max,SOME(j-2)))
 in
  if parenMatch parens then myBF(0,1,NONE)
  else NONE
 end