171. Excel Sheet Column Number

/*

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

*/

//思路：每次把数组循环一遍，减去ASCII码值64得数字，加起来就可以了。

//6ms  1.2%

int titleToNumber(char* s) {

    if(s[0]==NULL)

        return NULL;

        

    int length = strlen(s);

    int i,j,count = (s[length - 1] -64);

    for(i = length - 2, j = 1; i >= 0 ; i--,j++)

        count = count + (s[i] - 64)*pow(26,j);

    return count;

}