Word Abbreviation — C++

// Time:  O(n * l) ~ O(n^2 * l^2)
// Space: O(n * l)

class Solution {
public:
    vector<string> wordsAbbreviation(vector<string>& dict) {
        unordered_map<string, unordered_set<string>> abbr_to_word;
        unordered_map<string, string> word_to_abbr;

        for (const auto& word : dict) {
            const auto prefix = word.substr(0, 1);
            abbr_to_word[toAbbr(prefix, word)].emplace(word);
        }

        for (const auto& kvp : abbr_to_word) {
            if (kvp.second.size() > 1) {
                for (const auto& word : kvp.second) {
                    for (int i = 2; i < word.length(); ++i) {
                        const auto prefix = word.substr(0, i);
                        if (isUnique(prefix, kvp.second)) {
                            word_to_abbr[word] = toAbbr(prefix, word);
                            break;
                        }
                    }
                }
            } else {
                word_to_abbr[*kvp.second.begin()] = kvp.first;
            }
        }

        vector<string> result;
        for (const auto& word : dict) {
            result.emplace_back(word_to_abbr[word]);
        }
        return result;
    }

private:
    bool isUnique(const string& prefix, const unordered_set<string>& words) {
        return 1 == count_if(words.begin(), words.end(),
                             [&prefix](const string& word) {
                                 return !word.compare(0, prefix.length(), prefix);
                             });
    }

    string toAbbr(const string& prefix, const string& word) {
        string abbr = prefix;
        abbr += to_string(word.length() - 1 - prefix.length());
        abbr += word.back();
        return abbr.length() < word.length() ? abbr : word;
    }
};