今天朋友介绍了一个网站叫http://projecteuler.net/，非常有意思，其中有这么一道题：
Some positive integers n have the property that the sum [ n + 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 reverse(n).
There are 120 reversible numbers below one-thousand.
How many reversible numbers are there below one-billion (109)?
写了一段C程序，在我的机器上花了138秒求出了解，后来在朋友的指点下，优化成67秒。
跑去网贴上一看，有个牛人的程序更快，在我的机器上只要跑17秒，汗一个。