alien dictionary solution

Author: hyer0705

alien-dictionary/hyer0705.ts

@@ -0,0 +1,63 @@
+export class Solution {
+  alienOrder(words: string[]): string {
+    const indegree = new Map<string, number>();
+    const graph = new Map<string, Set<string>>();
+
+    for (const word of words) {
+      for (const ch of word) {
+        if (graph.has(ch)) continue;
+        graph.set(ch, new Set<string>());
+        indegree.set(ch, 0);
+      }
+    }
+
+    for (let i = 0; i < words.length - 1; i++) {
+      const word1 = words[i];
+      const word2 = words[i + 1];
+
+      let pointer = 0;
+      while (pointer < word1.length && pointer < word2.length && word1[pointer] === word2[pointer]) {
+        pointer++;
+      }
+
+      if (pointer < word1.length && pointer === word2.length) {
+        return "";
+      }
+
+      if (pointer < word1.length && pointer < word2.length) {
+        const neighbors = graph.get(word1[pointer])!;
+        if (!neighbors.has(word2[pointer])) {
+          neighbors.add(word2[pointer]);
+          indegree.set(word2[pointer], (indegree.get(word2[pointer]) || 0) + 1);
+        }
+      }
+    }
+
+    const queue: string[] = [];
+    const result: string[] = [];
+
+    for (const [ch, degree] of indegree) {
+      if (degree === 0) {
+        queue.push(ch);
+      }
+    }
+
+    while (queue.length > 0) {
+      const current = queue.shift()!;
+      result.push(current);
+
+      for (const neighbor of graph.get(current) || []) {
+        indegree.set(neighbor, (indegree.get(neighbor) || 0) - 1);
+        if (indegree.get(neighbor) === 0) {
+          queue.push(neighbor);
+        }
+      }
+    }
+
+    if (indegree.size === result.length) {
+      return result.join("");
+    }
+
+    return "";
+  }
+}