```Here is my solution, just uses backtracking and simple algorithm, no
complex strategics.

class Sodoku

def initialize( input )
@ary = Array.new(9) { Array.new(9) }  # 9 rows x 9 columns
index = 0
input.to_s.scan(/./) do |c|
case c
when '1'..'9' : @ary[index / 9][index % 9] = c.to_i
when '_'      : # skip
else next
end
break if (index += 1) == 9 * 9
end
# raise "Input data incomplete" if index != 9 * 9
end

def solve
9.times do |row|
9.times do |col|
next if @ary[row][col]

# find next possible value for @ary[row][col]
1.upto(9) do |v|
# check on same row/col, no any col/row already used it
next unless 9.times { |i| break if @ary[i][col] == v ||
@ary[row][i] ==v }

# check on 3x3 box, no other cell already used it
next unless 3.times do |i|
break unless 3.times do |j|
break if @ary[i + row / 3 * 3][j + col / 3 * 3] == v
end
end

@ary[row][col] = v

if solve
return true
else
@ary[row][col] = nil  # backtracking
end
end

return false
end
end
true
end

def to_s
s = "+-------+-------+-------+\n"
@ary.each_with_index do |row, index|
row = row.map { |e| e ||= '_' } * ' '
row[6,0] = row[12,0] = '| '
s << '| ' << row << " |\n"
s << "+-------+-------+-------+\n" if [2, 5, 8].include?(index)
end
s
end
end

if __FILE__ == \$0
input = ''
while line = gets
input << line
end

sodoku = Sodoku.new(input)
puts sodoku.solve ? sodoku : "No solution"
end

```