Reverse Words in a String

scan once, use stack

    void reverseWords(string &s) {
        stack<string> ret;
        int start;
        for(int i=0;i<s.length();i++){
            if(s[i]==' ') continue;
            start=i;
            while(i<s.length()&&s[i]!=' ') i++;
            ret.push(s.substr(start,i-start));
        }
        s="";
        if(!ret.empty()){
            s=ret.top();ret.pop();
        }
        while(!ret.empty()){
            s=s+' '+ret.top();ret.pop();
        }
    }