Some positive integers $n$ have the property that the sum $[n + \operatorname{reverse}(n)]$ consists entirely of odd (decimal) digits. For instance, $36 + 63 = 99$ and $409 + 904 = 1313$. We will call such numbers reversible; so $36$, $63$, $409$, and $904$ are reversible. Leading zeroes are not allowed in either $n$ or $\operatorname{reverse}(n)$.
There are $120$ reversible numbers below one-thousand.
How many reversible numbers are there below one-billion ($10^9$)?
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