Given an input string, reverse the string word by word.For example,Given s = "the sky is blue",return "blue is sky the".click to show clarification.Clarification:What constitutes a word?A sequence of non-space characters constitutes a word.Could the input string contain leading or trailing spaces?Yes. However, your reversed string should not contain leading or trailing spaces.How about multiple spaces between two words?Reduce them to a single space in the reversed string.

這道題很直接的想法就是用String的split函數(shù)，把字符串按空格分成一個個子字符串，存到一個String[] strs里面，這時再把strs里的字符串從大到小反轉(zhuǎn)連接，中間加上空格就好了。需要注意如果原來的字符串有multiple spaces, 那會造成我們strs數(shù)組里面有一些子字符串是空字符串，我們反轉(zhuǎn)時若遇到這些空字符串，就要把它跳過。時間上split操作是O(N)，再一次掃描獲得結(jié)果，也是O(N)。空間上使用了一個String[] 和StringBuffer，也是O（N）

本題就是需要注意一些語法，比如split函數(shù)的參數(shù)是String而不能是Char。我寫的時候不小心再一次犯了低級錯誤，String判斷是不是空字符串一定不要寫成if (str == ""), 要么用equals(), 要么用length()==0來判斷

注意 split()函數(shù)argument是一個string而不是char

 1 public class Solution { 2     public String reverseWords(String s) { 3         if (s==null || s.length()==0) return s; 4         s.trim(); 5         String[] strs = s.split(" "); 6         StringBuffer res = new StringBuffer(); 7         for (int i=strs.length-1; i>=0; i--) { 8             if (strs[i].length() == 0) continue; 9             res.append(strs[i]);10             res.append(' ');11         }12         return res.toString().trim();13     }14 }