Alien Dictionary solution

Author: PDKhan

alien-dictionary/PDKhan.cpp

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+class Solution {
+public:
+    string alienOrder(vector<string> &words) {
+        // Write your code here
+        unordered_map<char, unordered_set<char>> graph;
+        unordered_map<char, int> indegree;
+        unordered_set<char> all_chars;
+
+        for (const string& word : words) {
+            for (char c : word) {
+                all_chars.insert(c);
+                if (!indegree.count(c))
+                    indegree[c] = 0;
+            }
+        }
+
+        for(int i = 0; i < words.size() - 1; i++){
+            string word1 = words[i];
+            string word2 = words[i + 1];
+            int len = min(word1.size(), word2.size());
+            bool found = false;
+
+            for(int j = 0; j < len; j++){
+                char ch1 = word1[j];
+                char ch2 = word2[j];
+
+                if(ch1 != ch2){
+                    if(graph[ch1].count(ch2) == 0){
+                        graph[ch1].insert(ch2);
+                        indegree[ch2]++;
+                    }
+
+                    found = true;
+                    break;
+                }
+            }
+
+            if(!found && word1.size() > word2.size())
+                return "";
+        }
+
+        queue<char> q;
+        for(char c : all_chars){
+            if(indegree[c] == 0)
+                q.push(c);
+        }
+
+        string order;
+
+        while(!q.empty()){
+            char cur = q.front();
+            q.pop();
+
+            order += cur;
+
+            for(char next : graph[cur]){
+                if(--indegree[next] == 0)
+                    q.push(next);
+            }
+        }
+
+        if(order.size() != all_chars.size())
+            return "";
+
+        return order;
+    }
+};