Apply Substitutions Python Solution with Tests

lcpy gen -n 3481   # by problem number
lcpy gen -s apply_substitutions   # by problem name

Input: replacements = [["A","abc"],["B","def"]], text = "%A%_%B%"
Output: "abc_def"
Explanation: Replace %A% with "abc" and %B% with "def". The final text becomes "abc_def".

Input: replacements = [["A","bce"],["B","ace"],["C","abc%B%"]], text = "%A%_%B%_%C%"
Output: "bce_ace_abcace"
Explanation: Replace %A% with "bce" and %B% with "ace". Then substitute %B% in "abc%B%" with "ace" to obtain "abcace". The final text becomes "bce_ace_abcace".

class Solution:
    # Time: O(m + n * L) where m is total mapping size, L the average substituted length
    # Space: O(m + n * L)
    def apply_substitutions(self, replacements: list[list[str]], text: str) -> str:
        values: dict[str, str] = dict(replacements)

        def dfs(s: str) -> str:
            i = s.find("%")
            if i == -1:
                return s
            j = s.find("%", i + 1)
            if j == -1:
                return s
            key = s[i + 1 : j]
            return s[:i] + dfs(values[key]) + dfs(s[j + 1 :])

        return dfs(text)