翻转单词顺序列

public class Solution {
    public String ReverseSentence(String str) {
       if(str==null) {
           return null;
       }
        
        if(str.trim().equals(""))
        {
            return str;
        } 
        String string = str;
        String[] strings = string.split(" ");
        StringBuilder sbuilder = new StringBuilder();
        for(int i=strings.length-1;i>=0;i--)
        {
            if(i==0)
            {
                sbuilder.append(strings[i]);
            }
            else
            {
                sbuilder.append(strings[i]);
                sbuilder.append(" ");
            }
        }
        String string2 = sbuilder.toString();
        return string2;
    }
}