Given an input string, reverse the string word by word.

For example,

Given s = "the sky is blue",

return "blue is sky the".

Update (2015-02-12):

For C programmers: Try to solve it in-place in O(1) space.

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.

用栈即可解决，注意前导，尾随空格以及多个空格的问题。用正则表达式匹配多个空格然后用一个空格替换。String的split（“ ”）只能识别一个空格，之后的空格会按照字符串保存。

public class Solution {    public String reverseWords(String s) {        s=s.trim();        s=s.replaceAll("[\\s]+"," ");        String str[]=s.split(" ");        Stack
       
         sta=new Stack
        
         ();        String ans="";        for(String c:str){            sta.push(c);        }                while(!sta.isEmpty()){           ans=ans+sta.pop()+" ";        }                return ans.trim();    }}
        
       

public class StringAPIDemo08{    public static void main(String args[]){        String str1 = "hello   world" ;        // 定义字符串,3个空格        //str1=str1.replaceAll("[\\s]+", " ");        String s[] = str1.split(" ") ;        // 按空格进行字符串的拆分        System.out.println(s.length) ;      //s的长度为4，空格包含在了字符串里面。        for(int i=0;i