function alienOrder(words) {
  const adj = new Map();
  const inDegree = new Map();

  // Initialize all chars
  for (const word of words) {
    for (const ch of word) {
      if (!adj.has(ch)) adj.set(ch, new Set());
      if (!inDegree.has(ch)) inDegree.set(ch, 0);
    }
  }

  // Build edges from adjacent word comparisons
  for (let i = 0; i < words.length - 1; i++) {
    const w1 = words[i], w2 = words[i + 1];
    // Check invalid: w1 is longer and is a prefix of w2
    if (w1.length > w2.length && w1.startsWith(w2)) return "";
    for (let j = 0; j < Math.min(w1.length, w2.length); j++) {
      if (w1[j] !== w2[j]) {
        if (!adj.get(w1[j]).has(w2[j])) {
          adj.get(w1[j]).add(w2[j]);
          inDegree.set(w2[j], inDegree.get(w2[j]) + 1);
        }
        break;
      }
    }
  }

  // Topological sort (Kahn's)
  const queue = [];
  for (const [ch, deg] of inDegree) {
    if (deg === 0) queue.push(ch);
  }

  const result = [];
  while (queue.length > 0) {
    const ch = queue.shift();
    result.push(ch);
    for (const next of adj.get(ch)) {
      inDegree.set(next, inDegree.get(next) - 1);
      if (inDegree.get(next) === 0) queue.push(next);
    }
  }

  return result.length === inDegree.size ? result.join("") : "";
}