Given a column title as appear in an Excel sheet, return its corresponding column number.
For example:
A -> 1 B -> 2 C -> 3 ... Z -> 26 AA -> 27 AB -> 28
class Solution { public: int titleToNumber(string s) { int ret = 0; int help = 1; for (int i = s.size() - 1;i >= 0;--i) { ret += (s[i] - 'A' + 1)*help; help *= 26; } return ret; } };