Time Complexity to create a dictionary from a list in python

Question

What would be the time complexity to create a dictionary for frequency of occurrence of elements in a list (in python)?

For example: I have a list - ls = [5,3,3,2] I want to create a dictionary to check frequency of each element in the list. This is the code for that:

freq = {}
for ele in ls:
    if ele in freq:
        freq[ele] += 1
    else:
        freq[ele] = 1

This will create the dictionary from the list, but what would the time complexity be to create the dictionary?

Any insights on this would be helpful. Thank you.

Answer 1

It would be linear O(n) complexity as you are only iterating through the list once with no nested loops/recursion/etc... As for checking ele in freq it is O(1) complexity as dicts are hash maps and using dict.__contains__ only does a hash lookup for the value. Although using collections.Counter is the more preferred way:

from collections import Counter

ls = [5,3,3,2]
freq = Counter(ls)

---

>>> dict(freq)
{5: 1, 3: 2, 2: 1}

Answer 2

它应该是线性O(n)因为您只是从列表的第一个到最后一个线性循环。