Medium
重要的String method:
String str;
str.trim()
str.replaceAll(" +", " ");
還有StringBuilder的一個method也挺好用，叫sb.deleteCharAt();

public class Solution {
    public String reverseWords(String s) {
        s = s.trim().replaceAll(" +", " ");;
        if (s == null || s.length() == 0){
            return "";
        }
        String[] words = s.split(" ");
        StringBuilder sb = new StringBuilder();
        for (int i = words.length - 1; i >= 0; i--){
            sb.append(words[i]);
            sb.append(" ");
        }
        sb.deleteCharAt(sb.length() - 1);
        return sb.toString();
    }
}