[leetcode #171] Excel Sheet Column Number

Problem

Given a string columnTitle that represents the 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: columnTitle = "A"
Output: 1

Example 2:

Input: columnTitle = "AB"
Output: 28

Example 3:

Input: columnTitle = "ZY"
Output: 701

Constraints:

・ 1 <= columnTitle.length <= 7
・ columnTitle consists only of uppercase English letters.
・ columnTitle is in the range ["A", "FXSHRXW"].

Idea

A~Z까지의 알파벳으로 되어 있는 column title을 숫자로 변경하는 문제다.

알파벳의 수가 26개이므로 각 자리 수를 26진수로 변경하기만 하면 된다.

Solution

class Solution {
    public int titleToNumber(String columnTitle) {
        int factor = 1;
        int res = 0;
        for (int i=columnTitle.length()-1; i >= 0; i--) {
            res += factor * (columnTitle.charAt(i) - 'A' + 1);
            factor *= 26;
        }

        return res;
    }
}

Reference

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