Merge sort

def merge(list1, list2):
    res = []
    while len(list1) and len(list2):
        if list1[-1] > list2[-1]:
            res.append(list1.pop())
        else:
            res.append(list2.pop())
    res.reverse()
    return list1 + list2 + res

def merge_sort(arr):
    m = len(arr) // 2
    if m == 0:
        return arr
    return merge(merge_sort(arr[:m]), merge_sort(arr[m:]))