My implementation of merging two sorted lists in linear time - what could be improved?

Question

Fromg Google\'s Python Class:

E. Given two lists sorted in increasing order, create and return a merged
list of all the elements in sorted order. You may mod         


        

Answer 1

According to a note here:

# Note: the solution above is kind of cute, but unforunately list.pop(0)
# is not constant time with the standard python list implementation, so
# the above is not strictly linear time.
# An alternate approach uses pop(-1) to remove the endmost elements
# from each list, building a solution list which is backwards.
# Then use reversed() to put the result back in the correct order. That
# solution works in linear time, but is more ugly.    

and this link http://www.ics.uci.edu/~pattis/ICS-33/lectures/complexitypython.txt

append is O(1), reverse is O(n) but then it also says that pop is O(n) so which is which? Anyway I have modified the accepted answer to use pop(-1):

def linear_merge(list1, list2):
    # +++your code here+++
    ret = []
    while list1 and list2:
        if list1[-1] > list2[-1]:
            ret.append(list1.pop(-1))
        else:
            ret.append(list2.pop(-1))

    ret.reverse()

    return list1 + list2 + ret