Сортировка последовательности слов

#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
 
int main (){
        std::string line;
        std::getline (std::cin, line);
        std::vector<std::string> words;
        int a = 0;
        int b = line.length() - 2;
        for (a==1; a <=b; a += 4) {
        words.push_back (line.substr(a,3));
            }
 
std::sort (begin(words), end(words));
 
for (auto it = words.begin()+1; it!= words.end(); it++)
   if (*it == *(it-1) && *it != *(it+1))
   std::cout << *it << " ";
}

#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
 
int main (){
        std::string line;
        std::getline (std::cin, line);
        std::vector<std::string> words;
        int a = 0;
        int b = line.length() - 2;
        for (a=0; a <=b; a += 4) {
        words.push_back (line.substr(a,3));
            }
 
auto it = words.begin();
for (it; it != words.end(); ++it){
if ((*it == "end")&& (it != words.end()-1)){
words.erase (it, words.end());
break;
}
}
 
std::sort (begin(words), end(words)); 
 
for (auto it {words.begin()+1}; it!= words.end(); ++it)
   if (*it == *(it-1) && *it != *(it+1))
   std::cout << *it << " ";
}

for (auto it {words.begin()+1}; it!= words.end(); ++it)
  if (*it == *(it-1) && *it != *(it+1))