[剑指 Offer II 114. 外星文字典](拓扑排序)

​​剑指 Offer II 114. 外星文字典​​(拓扑排序)

先按照相邻单词比较建图。

然后跑拓扑排序。

class Solution {
public:
    unordered_map<char, vector<char>> edges;
    unordered_map<char, int> indegrees;
    bool valid = true;

    string alienOrder(vector<string>& words) {
        int length = words.size();
        for (auto word : words) {
            int wordLength = word.size();
            for (int j = 0; j < wordLength; j++) {
                char c = word[j];
                if (!edges.count(c)) {
                    edges[c] = vector<char>();
                }
            }
        }
        for (int i = 1; i < length && valid; i++) {
            addEdge(words[i - 1], words[i]);
        }
        if (!valid) {
            return "";
        }
        queue<char> qu;
        for (auto [u, _] : edges) {
            if (!indegrees.count(u)) {
                qu.emplace(u);
            }
        }
        string order;
        while (!qu.empty()) {
            char u = qu.front();
            qu.pop();
            order.push_back(u);
            for (char v : edges[u]) {
                indegrees[v]--;
                if (indegrees[v] == 0) {
                    qu.emplace(v);
                }
            }
        }
        return order.size() == edges.size() ? order : "";
    }

    void addEdge(string before, string after) {
        int length1 = before.size(), length2 = after.size();
        int length = min(length1, length2);
        int index = 0;
        while (index < length) {
            char c1 = before[index], c2 = after[index];
            if (c1 != c2) {
                edges[c1].emplace_back(c2);
                indegrees[c2] += 1;
                break;
            }
            index++;
        }
        if (index == length && length1 > length2) {
            valid = false;
        }
    }
};