带有编译错误的golang：未定义：strings.trim中的字符串

I try to use golang deal with this problem 557. Reverse Words in a String III my code as below:

import "fmt"
import  ss "strings"

func reverseWords(s string) string {
    words := ss.Split(s," ");
    res := "";
    for i:=0; i < len(words);i++{
        curWord := ss.Split(words[i],"");
        for j:=len(curWord)-1; j >= 0;j--{
            res += curWord[j];
        }
        if(i!=len(words)-1){
            res += " ";
        }
    }
    return res;
}

func main(){
    s := "Let's take LeetCode contest";
    fmt.Println(reverseWords(s));
}

Everything is ok in my pc, it can pass compile at least.
However, when I submit in leetcode it tell me :

 Line 67: undefined: strings in strings.Trim 

I google this error but get nothing relevant info. As a beginner in golang, I need help. Everything will be appreciated.