最近在用 recursive backtracking 写数独 (sudoku) solver，但调试的时候就出现stack level too deep (SystemStackError) 的问题，不知如何解决！

命令：ruby sudoku.rb < puzzle.txt

puzzle.txt 内容：

030090070
004800500
000001006
000000002
005406800
100000000
700200000
008003400
090070060

到 37 行是都会有这问题 以下是完整代码：

UNASSIGNED = 0
N = 9
problem = ''

while line = gets                               # read the txt file (puzzle)
  problem += line.chomp
end

problem = problem.split('').map(&:to_i).each_slice(9).to_a  # convert the puzzle into an 2-D integer array
grid = problem.dup

def FindUnassignedLocation(grid, row, col)
   0.upto(N-1) do |x|
    0.upto(N-1) do |y|
      if grid[x][y] == UNASSIGNED then return true end
    end
  end
  return false
end

def UsedInRow(grid, row, num)
  0.upto(N-1) do |col|
    if grid[row][col] == num then return true end
  end
  return false
end

def UsedInCol(grid, col, num)
  9.times do |row|
    if grid[row][col] == num then return true end
  end
  return false
end

def UsedInBox(grid, boxStartRow, boxStartCol, num)
  0.upto(2) do |row|
    0.upto(2) do |col|
      if grid[row+boxStartRow][col+boxStartCol] == num then
        return true
      end
    end
  end
  return false
end

def isSafe(grid, row, col, num)
  return !UsedInRow(grid, row, num) && !UsedInCol(grid, col, num) && !UsedInBox(grid, row - row%3, col - col%3, num)
end

def SolveSudoku(grid)
  row, col = 0, 0

  if !FindUnassignedLocation(grid, row, col) then
    return true
  end

  1.upto(9) { |num|                 # trying from 1 to 9
    if (isSafe(grid, row, col, num)) then
      grid[row][col] = num

      if SolveSudoku(grid) then             # stop when solved
        return true
      end

      grid[row][col] = UNASSIGNED       # undo and try again
    end
  }

    return false                    # this triggers backtracking
end

def printGrid(grid)
  grid.each {|s| p s}
end

if SolveSudoku(grid) == true then
  printGrid(grid)
else
  puts("No solution!")
end