How to Merge $k$ sorted lists (in Python 3)

The question comes from this nice blog on programming. I will solve it without giving much details, and test it quickly.

Solution

In [1]:
def merge_two(list1, list2):
    if len(list1) == 0:
        return list2[:]
    elif len(list2) == 0:
        return list1[:]
    else:
        if list1[0] < list2[0]:
            return [list1[0]] + merge_two(list1[1:], list2)
        else:
            return [list2[0]] + merge_two(list1, list2[1:])
    

def merge(*lists):
    head = []
    for list_i in lists:
        head = merge_two(head, list_i)
    return head

Tests

In [4]:
import random

def random_sorted_list(size):
    return sorted([random.randint(0, 100) for _ in range(size)])


def issorted(alist):
    return alist == sorted(alist) 


for size in [10, 20, 30]:
    for k in range(2, 20):
        lists = [random_sorted_list(size) for _ in range(k)]
        merged_list = merge(*lists)
        assert issorted(merged_list)

Complexity

One can prove that the algorithm we proposed is:

  • correctly merging $k$ sorted list into a sorted list containing the values from all the list,
  • and does so with an extra memory of at most $\mathcal{O}(k n)$ if all the lists have size at most $n$,
  • and does so with a time complexity of at most $\mathcal{O}(k n)$ if all the lists have size at most $n$.

Conclusion

Et voilà.