1002. Find Common Characters - Explanation

1. Frequency Count

class Solution:
    def commonChars(self, words: List[str]) -> List[str]:
        cnt = Counter(words[0])

        for w in words:
            cur_cnt = Counter(w)
            for c in cnt:
                cnt[c] = min(cnt[c], cur_cnt[c])

        res = []
        for c in cnt:
            for i in range(cnt[c]):
                res.append(c)

        return res

Time & Space Complexity

Time complexity: $O(n * m)$
Space complexity:
- $O(1)$ extra space, since we have at most $26$ different characters.
- $O(n * m)$ space for the output list.

Where $n$ is the number of words and $m$ is the length of the longest word.