翻转单词序列

public class Solution {
    public String ReverseSentence(String str) {
        int len=str.length();
        if(str==null || len==0)return "";
        if(str.indexOf(" ")==-1 ||str.trim().length()==0)return str;
        String[] temp = str.split(" ");
        str="";
        len=temp.length;
        while(len-->0){
            str= str+" "+temp[len];
        }
        return str.substring(1,str.length());
    }
}