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
