Python

Part 1 O(n log(n))

with open('day1.txt') as file:
  s = list(sorted(map(int,file)))

low = (x for x in reversed(s) if x <= 1010)
high = (x for x in s if x > 1010)

l = next(low)
h = next(high)

while True:
  if l + h > 2020:
    l = next(low)
  elif l + h < 2020:
    h = next(high)
  else:
    print(l,h,l+h,l*h)
    break

Part 2 O(n^2)

with open('day1.txt') as file:
  s = list(sorted(map(int,file)))

for y in s:
  low = (x for x in reversed(s))
  high = (x for x in s)

  l = next(low)
  h = next(high)
  try:
    while True:
      if y + l + h > 2020:
        l = next(low)
      elif y + l + h < 2020:
        h = next(high)
      else:
        print(y,l,h,y+l+h,y*l*h)
        break
  except StopIteration:
    continue