Leetcode Go Cells in a Range on an Excel Sheet

func cellsInRange(s string) []string {
    // pre allocate result
    res := make([]string, (s[3]-s[0]+1)*(s[4]-s[1]+1))
    var i int
    for c := s[0]; c <= s[3]; c++ {
        for r := s[1]; r <= s[4]; r++ {
            res[i] = string([]byte{c, r})
            i++
        }
    }
    
    return res
}