Python | Remove elements of list that are repeated less than k times

Given a list of integers (elements may be repeated), write a Python program to remove the elements that are repeated less than k times. Examples:

Input : lst = ['a', 'a', 'a', 'b', 'b', 'c'], k = 2
Output : ['a', 'a', 'a', 'b', 'b']

Input : lst = [1, 1, 1, 1, 2, 2, 3, 3, 3, 4, 4], k = 3
Output : [1, 1, 1, 1, 3, 3, 3]

Approach #1 : Pythonic naive Counter() from collections module construct a dictionary mapping values to counts and save them in ‘counted’. Then we make use of ‘temp_lst’ to store the elements that need to be removed. Finally, we traverse through the given list and append all elements that are not in ‘temp_lst’ to ‘res_lst’ containing the required output. 

Python3

# Python3 program to Remove elements of
# list that repeated less than k times
from collections import Counter

def removeElements(lst, k):
    counted = Counter(lst)

    temp_lst = []
    for el in counted:
        if counted[el] < k:
            temp_lst.append(el)

    res_lst = []
    for el in lst:
        if el not in temp_lst:
            res_lst.append(el)

    return(res_lst)

# Driver code
lst = ['a', 'a', 'a', 'b', 'b', 'c']
k = 2
print(removeElements(lst, k))

Output:

['a', 'a', 'a', 'b', 'b']

Time Complexity: O(N)

Auxiliary Space: O(N)

Approach #2 : Efficient Approach The efficient approach to use Counter method is to construct a dictionary mapping value to counts and then use a list comprehension to filter for counts larger than a specified value. This approach is both time and space efficient. 

Python3

# Python3 program to Remove elements of
# list that repeated less than k times
from collections import Counter

def removeElements(lst, k):
    counted = Counter(lst)
    return [el for el in lst if counted[el] >= k]

# Driver code
lst = ['a', 'a', 'a', 'b', 'b', 'c']
k = 2
print(removeElements(lst, k))

Output:

['a', 'a', 'a', 'b', 'b']

Time Complexity: O(N)

Auxiliary Space: O(N)