[leetcode]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 
    ...

Example 1:

Input: "A"
Output: 1

分析：

给定字母，返回相对应的数字。相当于26进制转10进制，从低位开始，依次为26的0次、1次、2次幂。。。再乘上(s[i]-'A'+1)即可。

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