from collections import deque
from typing import List, Deque


def merge(
    els1: List[int],
    els2: List[int],
) -> List[int]:
    """
    Merges two sorted lists into one, keeps order.
    >>> merge([1, 2, 3], [3, 4, 5])
    [1, 2, 3, 3, 4, 5]
    """
    result = []
    i, j = 0, 0
    while i < len(els1) and j < len(els2):
        if els1[i] < els2[j]:
            result.append(els1[i])
            i += 1
        else:
            result.append(els2[j])
            j += 1

    result.extend(els1[i:])
    result.extend(els2[j:])
    return result


def mergesort(elements: List[int]) -> List[int]:
    """
    Sorts a given list.
    """
    # -> if len(elements) == 0:
    # if not elements:
    if len(elements) <= 1:
        return elements

    buffer: Deque[List[int]] = deque()
    for el in elements:
        buffer.append([el])

    while len(buffer) > 1:
        h1: List[int] = buffer.popleft()
        h2: List[int] = buffer.popleft()
        buffer.append(merge(h1, h2))

    return buffer.pop()