Feat: 269. Alien Dictionary

Author: HC-kang

alien-dictionary/HC-kang.ts

@@ -0,0 +1,67 @@
+/**
+ * https://leetcode.com/problems/alien-dictionary
+ * T.C. O(N * M) N: number of words, M: average length of word
+ * S.C. O(1) 26 characters
+ */
+function alienOrder(words: string[]): string {
+  const graph: Record<string, Set<string>> = {};
+  const inDegree: Record<string, number> = {};
+
+  // Initialize the graph
+  for (const word of words) {
+    for (const char of word) {
+      if (!graph[char]) {
+        graph[char] = new Set();
+        inDegree[char] = 0;
+      }
+    }
+  }
+
+  // Build the graph
+  for (let i = 0; i < words.length - 1; i++) {
+    const word1 = words[i];
+    const word2 = words[i + 1];
+
+    // Check invalid case: if word1 is longer and is prefix of word2
+    if (word1.length > word2.length && word1.startsWith(word2)) {
+      return '';
+    }
+
+    let j = 0;
+    while (j < Math.min(word1.length, word2.length)) {
+      if (word1[j] !== word2[j]) {
+        const curSet = graph[word1[j]];
+        if (!curSet.has(word2[j])) {
+          curSet.add(word2[j]);
+          inDegree[word2[j]]++;
+        }
+        break;
+      }
+      j++;
+    }
+  }
+
+  // Topological sort
+  const queue: string[] = [];
+  for (const [char, degree] of Object.entries(inDegree)) {
+    if (degree === 0) {
+      queue.push(char);
+    }
+  }
+
+  const result: string[] = [];
+  while (queue.length) {
+    const char = queue.shift();
+    result.push(char!);
+    for (const next of graph[char!]) {
+      inDegree[next]--;
+      if (inDegree[next] === 0) {
+        queue.push(next);
+      }
+    }
+  }
+
+  return result.length === Object.keys(inDegree).length //
+    ? result.join('') 
+    : '';
+}