# mypy: ignore-errors
from typing import List, Optional


def two_sum(nums: List, target: int) -> Optional[int]:
    seen = {}
    for i, num in enumerate(nums):
        complement = target - num
        if complement in seen:
            return [seen[complement], i]
        seen[num] = i
    return None


# Example usage:
nums = [2, 7, 11, 15]
target = 9
result = two_sum(nums, target)
print(result)  # Output: [0, 1]