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

Example 2:

Input: "AB"
Output: 28

Example 3:

Input: "ZY"
Output: 701

int titleToNumber(string s) { // time: O(n); space: O(1)
    int res = 0;
    for (char c : s) {
        res = res * 26 + (c - 'A' + 1);
    }
    return res;
}