fillCells.py

available_options = (1..9)

def fillCells:
  doFillCells(0)
  
def doFillCells (index):
  cell = board.cells[index]
  neighbor_values = cell.neighbors.map((neighbor) => neighbor.values)
  remaining_options = available_options \ neighbor_values
  
  for option in remaining_options:
    cell.value = options
    
    if index == board.cells.count() - 1:
      return true
    if doFillCells(index + 1):
      return true
  
  # backtrack
  cell.value = 0
  return false