Reverse Words in a String

class Solution {
public:
    void reverseWords(string &s) {
        if(s.size()==0) return ;
        int i=0,j=s.size()-1;
        vector<string> res;
        while((i<=j)&&s[i]==' ')i++;
        while((i<=j)&&s[j]==' ')j--;
        while(i<=j)
        {
            int tmp=i;
            while((i<=j)&& s[i]!=' ')i++;
            res.push_back(s.substr(tmp,i-tmp));
            while((i<=j)&&s[i]==' ')i++;
        }
        s="";
        if(res.empty()) return;
        reverse(res.begin(),res.end());
        s=res[0];
        for(vector<string>::iterator it=++res.begin();it!=res.end();it++)
         s+=" "+*it;
    }
};