Excel Sheet Column Number - LeetCode javascript solutions

Problem

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

Solution

/**
 * @param {string} s
 * @return {number}
 */
var titleToNumber = function(s) {
  var res = 0;
  var num = 0;
  var len = s.length;
  for (var i = 0; i < len; i++) {
    num = getNum(s[len - 1 - i]);
    res += Math.pow(26, i) * num;
  }
  return res;
};

var getNum = function (char) {
  var start = 'A'.charCodeAt(0) - 1;
  return char.charCodeAt(0) - start;
};

Explain:

nope.

Complexity:

Time complexity : O(n).
Space complexity : O(1).