Python Counter application
\nimport collections\n\nclass Solution:\n    def maxFreq(self, s: str, maxLetters: int, minSize: int, maxSize: int) -> int:\n        shortest_substring = []\n  
```\nimport collections\n\nclass Solution:\n    def maxFreq(self, s: str, maxLetters: int, minSize: int, maxSize: int) -> int:\n        shortest_substring = []\n        for i in range(0, len(s) - minSize + 1):\n            shortest_substring.append(s[i:i+minSize])\n        counter = collections.Counter(shortest_substring)\n        res = 0\n        for k, v in counter.items():\n            if len(list(set(k))) <= maxLetters:\n                res = max(v, res)\n        return res\n            \n```
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