题目：hdu1013, hdu1163, 51nod1116.

or

dr(n) = (n-1)%9+1.

其中，n-1是为了将结果0-8匹配到1-9.

Reference:

[1] https://en.wikipedia.org/wiki/Digital_root

[2] https://en.wikipedia.org/wiki/Casting_out_nines