N Queens problem

a = [
    [0,0,0,0],
    [0,0,0,0],
    [0,0,0,0],
    [0,0,0,0]
]

def is_safe(a, row, col, n)
  #Check the same row
  (0).upto(n-1) do |i|
    if a[row][i] == 1
      return false
    end
  end

  i = row
  j = col
  while i >=0 && j >= 0
    if a[i][j] == 1
      return false
    end
    i -= 1
    j -= 1
  end

  i = row
  j = col

  while i <n  && j >= 0
    if a[i][j] == 1
      return false
    end
    i += 1
    j -= 1
  end

  true

end
def n_queens_problem(a, col, n)

  if col >= n
    return true
  end


  (0).upto(n-1) do |row|
    if is_safe(a, row, col, n)
      a[row][col] = 1
      puts a[row][col]

      if n_queens_problem(a, col+1, n)
        return true
      end

      #Back track
      a[row][col] = 0
    end
  end

  return false
end

puts n_queens_problem(a, 0,4)
puts a.to_s