Reverse Words in a String -- Leetcode

Question:

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.

Answer:

class Solution {

public:

    void reverseWords(string &s) {

        string ss;

        string res;

        

        for(int i=0; s[i]!='\0';++i){

            if(s[i]!=' '){

               ss.append(s[i],1);

            }

                 //For invalid ' ', if there exists duplicate ' '.

            else if(s[i] == ' ' && s[i+1] == ' '){

                continue;

            }

                 //For effective ' ', if there exists duplicate ' '. For case: "aa bb   cc   ".

            else if(s[i] == ' ' && s[i+1] != ' ' && s[i+1] != '\0'){

                res.insert(0," ");

                res.insert(1,ss);

                ss.clear();

            }

                 //For last word.

            else if(s[i] == ' ' && s[i+1] == '\0'){

                res.insert(0,ss);

                ss.clear();

            }

        }

        s = res;

    }

};