leetcode 171 Excel Sheet Column Number

Problem:
给一段字符串，然后求出它对应的数字，’A’-1,’AA’-27

class Solution {public:    int titleToNumber(string s) {        int ans = 0;        for(int i = 0; i < s.size(); i++) {            ans *= 26;            ans += s[i]-'A'+1;        }        return ans;    }};