LeetCode 127 Word Ladder - Finding Neighbors

Comparing every pair of words takes $O(N^2 \times L)$ time, where $N$ is the number of words and $L$ is word length. That is too slow for large inputs.

Instead, for each word, try changing each letter to every letter in the alphabet. For a word like hot, generate *ot, h*t, ho* by masking each position.

Check if these patterns match any word in the word list. Use a hash map from pattern to words. For hot, check if aot, bot,., zot exist, then hat, hbt,., hzt, and so on. This takes $O(L \times 26)$ per word, much faster than $O(N \times L)$.

Space complexity is $O(V)$ for the data structures used.