leetcode557+ reverse string+使用stack，注意最后一个string的空格问题

https://leetcode.com/problems/reverse-words-in-a-string-iii/description/

class Solution {
public:
    string reverseWords(string s) {
        stack<char> st;
        string str="";
        for(int i=0;i<s.size();i++){
            if(s[i]==' '){
                while(!st.empty()){
                    str+=st.top();
                    st.pop();
                }
                str += ' ';
            }
            else st.push(s[i]);
        }
        while(!st.empty()){
            str += st.top();
            st.pop();
        }
        return str;
    }
};