LeetCode Reverse Words in a String

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.

给定一个字符串，将字符串中的英文单词反转过来：

实现一：以单词作为整体的方式，实现

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

 实现二：遍历字符串中的每一个字符来实现

public class Solution {
    public String reverseWords(String s) {
		 if(s==null)
			 return s;
		 Stack stack = new Stack();
		 int i=s.length()-1;
		 StringBuffer sb =new StringBuffer();
		 while(i>=0)
		 {
			 char c = s.charAt(i);
			 if(c==' ')
			 {
				 i--;
				 while(i>=0&&s.charAt(i)==' ')
					 i--;
				 
				 if(!stack.empty())
				 {
					 while(!stack.empty())
					 {
						 char ch = (Character) stack.pop();
						 sb.append(ch);
					 }
					 sb.append(" ");
					 stack = new Stack();
				 }
			 }
			 else{
				 i--;
				 stack.push(c);
			 }
		 }
		 if(!stack.empty())
		 {
			 while(!stack.empty())
			 {
				 char ch = (Character) stack.pop();
				 sb.append(ch);
			 }
		 }
		 if(sb.length()>0&&sb.charAt(sb.length()-1)==' ')
		 sb.deleteCharAt(sb.length()-1);
		 return sb.toString();
	 }
}