leetcode - Reverse Words in a String

class Solution {
public:
void reverseWords(string &);
};
void Solution::reverseWords(string &s){
int len = s.length();
int i =0 ;
stack<string> st;
while (i<len){
while (i<len && s[i]==' ')
i++;
string tmpStr="";
while (i<len && s[i] != ' '){
tmpStr += s[i];
i++;
}
if (tmpStr.length()>0)
st.push(tmpStr);
}
s = "";
while (!st.empty()){
if (s.length()>0)
s += " ";
s += st.top();
st.pop();
}
}