数学

power of two   http://www.cnblogs.com/-1307/p/6907811.html

Add Digits   https://leetcode.com/problems/add-digits/#/solutions

各位数相加

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 ……

0 1 2 3 4 5 6 7 8 9  1   2   3   4   5 ……

=>  1 + （n-1）%9

Island Perimeter   https://leetcode.com/problems/island-perimeter/#/description