Python DP `O(1)` space, super EASY.
python\nclass Solution:\n    def countVowelStrings(self, n: int) -> int:\n        a, e, i, o, u = 1, 1, 1, 1, 1\n        for _ in range(n):\n            a, e, i
```python\nclass Solution:\n    def countVowelStrings(self, n: int) -> int:\n        a, e, i, o, u = 1, 1, 1, 1, 1\n        for _ in range(n):\n            a, e, i, o, u = a+e+i+o+u, e+i+o+u, i+o+u, o+u, u\n        return a\n```
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