excel sheet column number

Question:
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 

Approach

A good way to go about this is to write out some examples. There are already some examples, but we should write out some for 3 char length like AAA. Then we can try to see if there is some algorithm to calculate them. Once we come up with an algorithm, writing the code should be a lot easier.

A : 1
B : 2
Z : 26
AA :27 - > 1 + 1*26
AB :28 - > 2 + 1*26
BA :53 - > 1 + 2*26
BB :54 - > 2 + 2*26
ZA :677 -> 1 + 26*26
ZZ :702 -> 26 + 26*26
AAA : 703 -> 1 + 27*26 - > 1 + 1*26 + 1*26*26

It took me a while to realize that you just have to use 26^x to get the values. Using the above data we can derive a function to calculate the number for any cell location. We start at the least significant char and multiply its int value by 26^x where x = the ith position in the char starting from the right.

We can loop through the string going backwards just adding to the sum and incrementing the power on each loop. We create a string to store the alphabet so we can convert single characters to their int value.

Solution

var titleToNumber = function(s) {
    var alphabet = "|ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    var sum = 0;
    var power = 0;
    for(var i = s.length - 1; i >= 0; i--) {
        sum += alphabet.indexOf(s[i]) * Math.pow(26,power);
        power++;
    }
    return sum;
};