The brute-force solution is the correct one in this case. Otherwise, you can
iterate through palindromes instead of searching the range directly.

Note there are only 2 non-trivial palindromes in base 2 and 3 simultaneously 
in the input range: 6,643 and 1,422,773. There are no palindromes in base 2, 3, and
4 simultaneously in the input range.