Excel Sheet Column Number - LeetCode 171

题目描述：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

分析：所有大写字母A-Z，依次映射到1-26。然后AA = 1 * 26 + 1，AB = 1 * 26 = 2，等等，不难看出，这其实就是26进制的换算，不多说了，直接给出C++实现代码：

class Solution {
public:
int titleToNumber(string s) {
int num= 0;
string::size_type i = 0;
for( i = 0; i < s.size(); i++ )
{
num = num * 26 + (s[i]-'A' + 1);    /* 该处直接利用字符的ASCII码计算 */
}
return num;
}
};