127. Word Ladder

1function ladderLength(beginWord: string, endWord: string, wordList: string[]): number {
2  const dict = new Set(wordList);
3  if (!dict.has(endWord)) return 0;
4  let front = new Set([beginWord]), back = new Set([endWord]), steps = 1;
5  dict.delete(endWord);
6  while (front.size && back.size) {
7    if (front.size > back.size) [front, back] = [back, front];
8    const next = new Set<string>();
9    for (const w of front)
10      for (let i = 0; i < w.length; i++)
11        for (const c of "abcdefghijklmnopqrstuvwxyz") {
12          const v = w.slice(0, i) + c + w.slice(i + 1);
13          if (back.has(v)) return steps + 1;
14          if (dict.delete(v)) next.add(v);
15        }
16    front = next; steps++;
17  }
18  return 0;
19}

One frontier from each end.

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