Cells in a Range on an Excel Sheet

class Solution {
    public List<String> cellsInRange(String s) {
        String[] strs = s.split(":");
        String start = strs[0];
        String end = strs[1];
        List<String> res = new ArrayList<>();
        for(char a = start.charAt(0); a <= end.charAt(0); a++){
            for(char b = start.charAt(1); b <= end.charAt(1); b++){
                res.add(""+a+b);
            }
        }
        return res;
    }
}