LEETCODE : 171. Excel Sheet Column Number

LEETCODE : 171. Excel Sheet Column Number

Link : https://leetcode.com/problems/excel-sheet-column-number/

Problem Description : 

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 

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Explaination : 

Initially it looks like a hard problem but when we get to know the logic behind it then it seems to be a really easy problem.
The observation needed here is that, the column title value represents a number of base 26. And all we need to do is to convert that base 26 number to base 10 number.

To convert the base to base 10, we need to do what we usually do for binary or octal converstions.

For example : 

BAD

B=2
A=1
D=4

  2x(26)^2 + 1x(26) + 4x(26)^0 
= 1352 + 26 + 4
= 1382

In C++ we can write it as below :

class Solution {
public:
    int titleToNumber(string s) {
        int ans =0;
        long pwr=1;
        reverse(s.begin(), s.end()) ;
        for(auto c:s){
            ans+=(pwr*(c-'A' +1));
            pwr=pwr*26;
        }
        return ans;
    }
};