[leetcode 151]Reverse Words in a String

    public String reverseWords(String s){    
        s = s.trim();
        String[] words = s.split(" ");
        
        ArrayList<String> arrayList = new ArrayList<String>();
        for (String string : words) {
            if (!string.isEmpty()) {
                arrayList.add(string);
            }
        }
        if (arrayList.isEmpty()) {
            return "";
        }
        
        int len = arrayList.size();
        
        StringBuffer sb = new StringBuffer();
        for (int i = 0; i < len - 1; i++) {//少加一个，避免最后一个单词后面有空格
            sb.append(arrayList.get(len - i - 1));
            sb.append(" ");
        }
        sb.append(arrayList.get(0));
    
        return sb.toString();
    }