LeetCode Excel Sheet Column Number

int titleToNumber(char* s) {
int length=strlen(s),num=0,i=0;
while(i<length)
{
num=num+(s[i]-64)*pow(26,length-i-1);//方法一
//num=s[i]-64+num*26;//方法二
i++;
}
return num;
}

double pow(double x, double y);