First missing positive number in an array

first_missing_positive_num.py

class Solution:
    def firstMissingPositive(self, nums: List[int]) -> int:
        # filter non negative, new size n
        arr = sorted(filter(lambda x: x > 0, nums))
        count = len(arr)
        # filter all num <= n to a hash set and keep new count, count.
        arr = filter(lambda x: x <= count, set(arr))
        greater_positive_num = 1
        for num in arr:
            if greater_positive_num == num:
                greater_positive_num += 1
            else:
                break
        return greater_positive_num