5242. Greatest English Letter in Upper and Lower Case

class Solution:
    def greatestLetter(self, s: str) -> str:
        count = {}
        res = ''
        for i in range(len(s)):
            if count.get(s[i].upper(), [0, 0]) == [1, 1]:
                continue
            lower, upper = count.get(s[i].upper(), [0, 0])
            
            if lower == 0 and s[i].islower():
                count[s[i].upper()] = [1, upper]
                
            if upper == 0 and s[i].isupper():
                count[s[i].upper()] = [lower, 1]
                
        for c in count:
            if count[c] == [1, 1]:
                res = max(res, c)
        return res