LeetCode: 171 Excel Sheet Column Number(easy)

题目：

Related to question Excel Sheet Column Title

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 

代码：

 class Solution {
 public:
     int titleToNumber(string s) {
         map<char, int> map;
         int num = ;
         int result = ;
         for (char i = 'A'; i<='Z'; i++){
             map[i] = num;
             num++;
         }
         int j = ;
         for(auto c = s.crbegin(); c<s.crend(); c++){
             result = map[*c]*pow(, j) + result;
             j++;
         }

         return result;
     }
 };