class Solution {
   public String reverseWords(String s){
        //1.去掉首尾空格
         String S= s.trim();
         StringBuilder res=new StringBuilder();
        //双指针倒序遍历字符串，j锁定尾部，i去找头部
        int i=S.length()-1;
        int j=i;
        while(i>=0){
            while(i>=0&&S.charAt(i)!=' ')i--;
            res.append(S.substring(i+1,j+1)+' ');
            while(i>=0&&S.charAt(i)==' ') i--;
            j=i;
        }
        return res.toString().trim();

    }
}