Reverse Words in a String

https://leetcode.com/problems/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".

Update (2015-02-12):
For C programmers: Try to solve it in-place in O(1) space.

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.

import java.util.ArrayList;
import java.util.List;
import java.util.Stack;

public class Solution {
    public static String reverseWords(String s) {
    List<String> li=new ArrayList();
        int len0=s.length();
        int len1=0;
        String x="";
        for(int i=0;i<len0;i++){
            char c=s.charAt(i);
            if(c!=' ') x+=c;
            else{
            if(x.length()!=0){
                len1++;
                li.add(x);
            }
            x="";
            }
        }
        if(x.length()!=0) li.add(x);
        Stack<String>st=new Stack();
        for(String str:li){
            st.add(str);
        }
        s="";
        while(!st.isEmpty()){
            s+=st.pop();
            if(st.size()!=0)
            s+=" ";
        }
        return s;
    }
    public static void main(String[]args){
    System.out.println(reverseWords("   a   b "));
    }
}