171. Excel Sheet Column Number.cpp

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
Example 2:

Input: "AB"
Output: 28
Example 3:

Input: "ZY"
Output: 701


Constraints:

1 <= s.length <= 7
s consists only of uppercase English letters.
s is between "A" and "FXSHRXW".








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