go + 双指针

func solve( str string ) string {
    // write code here
    if len(str) == 0 {
        return ""
    }

    i, j := 0, len(str)-1
    bs := []byte(str)
    for ; i< j; i, j = i+1, j-1 {
        bs[i], bs[j] = bs[j], bs[i]
    }

    return string(bs)
}