words. Find every letter that appears in all of the words, counting repeats: if a letter occurs at least k times in each word, it must appear exactly k times in the answer.
Return these letters (each as a one-character string) in any order.words=['banana', 'cabana', 'anagram'] → ['a', 'a', 'a', 'n']
The letter a appears at least three times in every word, and n at least once; b is missing from one word.
words=['sun', 'moon'] → ['n']
Only n is shared, once.
words=['dog', 'cat'] → []
words=['banana', 'cabana', 'anagram']
['a', 'a', 'a', 'n']
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