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.

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

#include <iostream>
#include <string>
#include <algorithm>
#include <sstream>
#include <vector>

using namespace std;

void reverseWords(string &s){
    string buf;
    stringstream ss(s);
    vector<string> tokens;
    while (ss >> buf) tokens.push_back(buf);
    if (tokens.size() == 0)  s="";
    else{
        int n = tokens.size()-1;
        s = tokens[n];
        for (int i = n-1; i >=0; -- i) s+=" "+tokens[i];
    }
}

int main(){
    string s = "the sky is blue";
    reverseWords(s);
    cout<<s<<endl;
}