[LeetCode] 171. Excel Sheet Column Number

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

class Solution {
    public int titleToNumber(String s) {
        Map map = new HashMap<>();
        int i = 1;
        for (char ch = 'A'; ch <= 'Z'; ch++) {
            map.put(ch, i++);
        }
        int res = 0;
        for (char ch: s.toCharArray()) {
            res = res*26+map.get(ch);
        }
        return res;
    }
}