reverse words in a string in leetcode

leetcode-reverse_words

#include <stack>
#include <string>
using namespace std;

class Solution {
public:
    void reverseWords(string &s) {
        stack<string> stk;
        int i=0;
        int left=0;
        int right=-1;
        while(i<s.length()){
            if(s.at(i)==' '){
                left=right+1;
                right=i;
                stk.push(s.substr(left,right-left));
                // cout<<s.substr(left,right-left)<<endl;
            }
            i++;
        }
        stk.push(s.substr(right+1,i-right));
        // cout<<s.substr(right+1,i-right)<<endl;
        int k=0;
        while(!stk.empty()){
        	string ss=stk.top();
        	stk.pop();
        	// cout<<ss.c_str()<<endl;
        	int tmp=0;
        	while(tmp<ss.length()){
        		s[k++]=ss.at(tmp);
        		tmp++;
        	}
        	if(!stk.empty())
        		s[k++]=' ';
        }
    }
};

leetcode give a erroe info: "output limit exceeded"

line 21 cause wrong when input string is " ".