wave_list.py

#!/usr/bin/python3

import random

# The program sorts the list of numbers in the form of waves.
# It means a list for which the following condition
# is true a1> = a2 <= a3> = a4 <= ...

def merge_sort(l):
    """Recursive merge sort."""
    if len(l) > 1:
        mid = len(l) // 2
        left = l[:mid]
        right = l[mid:]

        merge_sort(left)
        merge_sort(right)

        i = j = k = 0
        while i < len(left) and j < len(right):
            if left[i] < right[j]:
                l[k] = left[i]
                i += 1
            else:
                l[k] = right[j]
                j += 1
            k += 1

        if i < len(left):
            l[k: k + len(left[i:])] = left[i:]
        if j < len(right):
            l[k: k + len(right[j:])] = right[j:]

n = int(input('Enter the length of the list (positive integer):\n >'))
lst = random.sample(range(n * 10), n)
print("Input list: {}".format(lst))

merge_sort(lst)

arr = []
i = 0
tmp = 0

while i != n - 1:
    if i % 2 == 0:
        tmp = lst[i]
        arr.append(lst[i+1])
    else:
        arr.append(tmp)
    i += 1

print("Output list: ", arr)