leetcode (go language) converted to lowercase

// tolowercase
package main

import (
    "fmt"
)

func isValidAlph(s string) bool {
    return s >= "A" && s <= "Z"
}

func toLowerCase(str string) string {
    ret := ""
    for _, value := range str {
        ch := fmt.Sprintf("%c", value)
        if isValidAlph(ch) {
            value = value + 'a' - 'A'
            ret += fmt.Sprintf("%c", value)
        } else {
            ret += ch
        }
    }
    return ret
}

func main() {
    fmt.Println(toLowerCase("He9llo"))
}

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