Easy with golang
\nfunc greatestLetter(s string) string {\n\tlowerArr, upperArr := make([]byte, 26), make([]byte, 26)\n\n\tfor i := 0; i < len(s); i++ {\n\t\tif s[i] >= \'a\' &&
```\nfunc greatestLetter(s string) string {\n\tlowerArr, upperArr := make([]byte, 26), make([]byte, 26)\n\n\tfor i := 0; i < len(s); i++ {\n\t\tif s[i] >= \'a\' && s[i] <= \'z\' {\n\t\t\tlowerArr[s[i]-\'a\']++\n\t\t} else {\n\t\t\tupperArr[s[i]-\'A\']++\n\t\t}\n\t}\n\n\tfor i := 25; i >= 0; i-- {\n\t\tif lowerArr[i] > 0 && upperArr[i] > 0 {\n\t\t\treturn fmt.Sprintf("%c", \'A\'+i)\n\t\t}\n\t}\n\n\treturn ""\n}\n\n```
['Go']