【easy】168. Excel Sheet Column Title 171. Excel Sheet Column Number

class Solution {
public:
    string convertToTitle(int n) {
        if (n == ) {
            return "";
        }
        return convertToTitle((n - ) / ) + (char)((n - ) %  + 'A');
    }
};

    A -> 1
    B -> 2
    C -> 3
    ...
    Z -> 26
    AA -> 27
    AB -> 28 
答案写法：

class Solution {
public:
    int titleToNumber(string s) {

        int sum = ;
        int tmp = ;
        for (int i = ; i < s.length(); ++i) {
            tmp = s[i] - 'A' + ;
            sum =  * sum + tmp;
        }
        return sum; 
　　}
};

下面是自己写的：

class Solution {
public:
    int titleToNumber(string s) {
        int len = s.length();
        int num = ;
        if (len == )
            num = s[]-'A'+;
        else{
            num += s[len-]-'A'+;
            int cal = ;
            for (int i=len-;i>=;i--){
                num += (s[i]-'A'+)*cal;
                cal *= ;
            }

        }
        return num;
    }
};//从字符串到数字