171. Excel Sheet Column Number

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 
本质：26进制转换为10进制
以下这个例子是错误的，因为比如AB，按字符串的顺序排在第一位的是A而不是B

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

正确的是：

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

这样也是对的

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