171. Excel表列序号

 1 class Solution 
 2 {
 3 public:
 4     int titleToNumber(string s) 
 5     {
 6         unordered_map<char,int> hash;
 7         for(int i = 1;i <= 26;i ++) hash[‘A‘ + i - 1] = i;
 8         int res = 0;
 9         int n = s.size();
10         for(int i = n - 1;i >= 0;i--) res += hash[s[i]] * pow(26,n - i - 1);
11         return res;
12     }
13 };