Extract longest palindrome from a string of letters

longest-palindome.cc

#include <iostream>
#include <string>

std::string longest_palindrome(const std::string& word){
    if(word.size()<3)
      return std::string();
    std::string longest("");
    
    for(std::size_t i=1;i<word.size()-1;++i){
      auto left = i - 1;
      auto right= i + 1;
      if(word[left] == word[right]){
        while(left >= 0 && right < word.size()){
          if(word[left] != word[right]){//reached end of palindrome
            break;
          }
          left--;right++;
        }
        //extract palindrome
        auto p = word.substr(left+1,(right-1)-(left+1)+1);
        if(longest.size()< p.size()){
          longest = p;
        }
      }
    }
    return longest;
}

int main() {

    std::cout << longest_palindrome("") << std::endl;
    std::cout << longest_palindrome("aa") << std::endl;
    std::cout << longest_palindrome("aaa") << std::endl;
    std::cout << longest_palindrome("ama") << std::endl;
    std::cout << longest_palindrome("madam") << std::endl;
    std::cout << longest_palindrome("zzzmadam") << std::endl;
    std::cout << longest_palindrome("madamzzz") << std::endl;
    std::cout << longest_palindrome("aibohphobia") << std::endl;    
}