[LeetCode] Alien Dictionary

 1 class Solution {
 2 public:
 3     string alienOrder(vector<string>& words) {
 4         if (words.size() == 1) return words[0];
 5         graph g = make_graph(words);
 6         return toposort(g);
 7     }
 8 private:
 9     typedef unordered_map<char, unordered_set<char>> graph;
10     
11     graph make_graph(vector<string>& words) {
12         graph g;
13         int n = words.size();
14         for (int i = 1; i < n; i++) {
15             bool found = false;
16             string word1 = words[i - 1], word2 = words[i];
17             int m = word1.length(), n = word2.length(), l = max(m, n);
18             for (int j = 0; j < l; j++) {
19                 if (j < m && g.find(word1[j]) == g.end())
20                     g[word1[j]] = unordered_set<char>();
21                 if (j < n && g.find(word2[j]) == g.end())
22                     g[word2[j]] = unordered_set<char>();
23                 if (j < m && j < n && word1[j] != word2[j] && !found) {
24                     g[word1[j]].insert(word2[j]);
25                     found = true;
26                 }
27             }
28         }
29         return g;
30     }
31     
32     string toposort(graph& g) {
33         unordered_set<char> path, visited;
34         string topo;
35         for (auto adj : g)
36             if (!acyclic(g, path, visited, topo, adj.first))
37                 return "";
38         reverse(topo.begin(), topo.end());
39         return topo;
40     }
41     
42     bool acyclic(graph& g, unordered_set<char>& path, unordered_set<char>& visited, string& topo, char node) {
43         if (path.find(node) != path.end()) return false;
44         if (visited.find(node) != visited.end()) return true;
45         path.insert(node); visited.insert(node);
46         for (auto neigh : g[node])
47             if (!acyclic(g, path, visited, topo, neigh))
48                 return false;
49         path.erase(node);
50         topo += node;
51         return true;
52     }
53 };