sudoku.nt

module sudoku;

import std.util, std.coroutine, std.string, std.fun;

auto solve(string puz){
alias s=string,v=void delegate(s);
void S(s p,v y){
if (auto i=p.find"0"){p=p.dup;for int k<-0..9 if!sum$(0..81).select(λ(int j)->(i%9==j%9||i/9==j/9||i/27==j/27&&(i%9)/3==(j%9)/3))
.map(λ(int i)->(p[i]=="1"+k)).eval{p[i]="123456789"[k];p.S y;}}else y p;}
return setupCoIterator!s new λ(v y)->puz.S y;
}

void main() {
  string first <- cat("0" x 81).eval.solve();
  writeln first;
  writeln "--";
  auto puz = "027800061000030008910005420500016030000970200070000096700000080006027000030480007";
  for string s <- puz.solve() writeln s;
}

--output--

123456789456789123789123456214365897365897214897214365531642978642978531978531642
--
327894561465132978918765423589216734643978215172543896794651382856327149231489657
327894561465132978918765423589216734643978215271543896794651382856327149132489657
327894561645132978918765423589216734463978215172543896794651382856327149231489657
327894561645132978918765423589216734463978215271543896794651382856327149132489657