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Alien Dictionary

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Description

https://leetcode.com/problems/alien-dictionary/description/

Solution

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class Solution {
public:
    void extractRules(string& word1, string& word2, unordered_map<char, unordered_set<char>>& rules, unordered_map<char, int>& counter) {
        int index = 0;
        int bound = min(word1.size(), word2.size());
        while(index < bound && word1[index] == word2[index]) {
            ++index;
        }
        if (index == bound) return;
        char left = word1[index];
        char right = word2[index];
        
        if(rules[left].find(right) == rules[left].end()) {
            rules[left].insert(right);
            ++counter[right];
        }
    }
    
    void fillCharacter(unordered_map<char, int>& counter, vector<string>& words) {
        for (string& word : words) {
            for(int index = 0; index < word.size(); ++index) {
                if (counter.count(word[index]) == 0) counter[word[index]] = 0;
            }   
        }
    }
    
    string alienOrder(vector<string>& words) {
        int size = words.size();
        string ret;
        if (size == 0) return ret;
        unordered_map<char, unordered_set<char>> rules;
        unordered_map<char, int> counter;
        
        fillCharacter(counter, words);
        
        for (int i = 0; i < size - 1; ++i){
            for (int j = i + 1; j < size; ++j) {
                string& word1 = words[i];
                string& word2 = words[j];
                extractRules(word1, word2, rules, counter);
            }
        }
        
        queue<char> que;
        for (auto iter = counter.begin(); iter != counter.end(); ++iter) {
            if (iter->second == 0) {
                ret += iter->first;
                que.push(iter->first);   
            }
        }
    
        
        while (!que.empty()) {
            char c = que.front();
            que.pop();
            unordered_set<char>& follows = rules[c];
            for (char ch : follows) {
                --counter[ch];
                if (counter[ch] == 0) {
                    ret += ch;
                    que.push(ch);
                }
            }
        }
        
        for (auto& iter : counter) if (iter.second != 0) return string();
    
        return ret;
    }
};