day7.py

# no runtime given, but O(n)
class Solution:
    def countElements(self, arr: List[int]) -> int:
        seen = set(arr)
        return sum([1 if n+1 in seen else 0 for n in arr])

# 28ms with reduce; also O(n)
class Solution:
    def countElements(self, arr: List[int]) -> int:
        seen = set(arr)
        return reduce(lambda accumulated, x: accumulated + (1 if x+1 in seen else 0), arr, 0)

# this one seems the fastest; didn't know you can sum over a generator directly without an iterable
class Solution:
    def countElements(self, arr: List[int]) -> int:
        seen = set(arr)
        return sum(1 if n+1 in seen else 0 for n in arr)