Python program to Sort a List of Strings by the Number of Unique Characters

Given a list of strings. The task is to sort the list of strings by the number of unique characters.

Examples:

Input : test_list = [‘gfg’, ‘best’, ‘for’, ‘geeks’], 
Output : [‘gfg’, ‘for’, ‘best’, ‘geeks’] 
Explanation : 2, 3, 4, 4 are unique elements in lists.

Input : test_list = [‘gfg’, ‘for’, ‘geeks’], 
Output : [‘gfg’, ‘for’, ‘geeks’] 
Explanation : 2, 3, 4 are unique elements in lists. 

Method #1 : Using sort() + len() + set()

In this, we perform task of sorting using sort(), and len and sort functions are used to get length of unique characters in string.

Python3

# Python3 code to demonstrate working of
# Sort Strings by Unique characters
# Using sort() + len() + set()

# helper function
def hlper_fnc(ele):

    # getting Unique elements count
    return len(list(set(ele)))

# initializing list
test_list = ['gfg', 'best', 'for', 'geeks']

# printing original list
print("The original list is : " + str(test_list))

# perform sort
test_list.sort(key = hlper_fnc)

# printing result
print("Sorted List : " + str(test_list))

Output

The original list is : ['gfg', 'best', 'for', 'geeks']
Sorted List : ['gfg', 'for', 'best', 'geeks']

Time Complexity: O(nlogn)

Space Complexity: O(n)

Method #2 : Using sorted() + len() + set() + lambda

Similar to above method, difference being not inplace sort, and also uses lambda function for performing task. 

Python3

# Python3 code to demonstrate working of
# Sort Strings by Unique characters
# Using sorted() + len() + set() + lambda

# initializing list
test_list = ['gfg', 'best', 'for', 'geeks']

# printing original list
print("The original list is : " + str(test_list))

# perform sort
res = sorted(test_list, key = lambda sub : len(list(set(sub))))

# printing result
print("Sorted List : " + str(res))

Output

The original list is : ['gfg', 'best', 'for', 'geeks']
Sorted List : ['gfg', 'for', 'best', 'geeks']

Time Complexity: O(nlogn)

Space Complexity: O(n)

Python3

# Python3 code to demonstrate working of
# Sort Strings by Unique characters
# Using sort() + len() + set()

# helper function


def hlper_fnc(ele):

    # getting Unique elements count
    return len(list(set(ele)))


# initializing list
test_list = ['gfg', 'best', 'for', 'geeks']

# printing original list
print("The original list is : " + str(test_list))

# perform sort
test_list = sorted(test_list, key=hlper_fnc)

# printing result
print("Sorted List : " + str(test_list))

Output

The original list is : ['gfg', 'best', 'for', 'geeks']
Sorted List : ['gfg', 'for', 'best', 'geeks']

Method 4:Using Counter() function

Python3

# Python3 code to demonstrate working of
# Sort Strings by Unique characters

from collections import Counter

# helper function


def hlper_fnc(ele):
  # getting Unique elements
    freq = Counter(ele)
    # getting Unique elements count
    return len(freq)


# initializing list
test_list = ['gfg', 'best', 'for', 'geeks']

# printing original list
print("The original list is : " + str(test_list))

# perform sort
test_list.sort(key=hlper_fnc)

# printing result
print("Sorted List : " + str(test_list))

Output

The original list is : ['gfg', 'best', 'for', 'geeks']
Sorted List : ['gfg', 'for', 'best', 'geeks']

Time Complexity: O(nlogn)

Auxiliary Space: O(n)

Method 5: Using NumPy

Here’s an approach using NumPy, which creates a NumPy array from the input list and then sorts the array based on the number of unique characters in each string.

Python3

import numpy as np

def sort_list_unique_chars(lst):
    # Create a NumPy array from the input list
    arr = np.array(lst)

    # Get the length of unique characters in each string
    unique_char_count = np.array([len(np.unique(i)) for i in arr])

    # Sort the NumPy array based on the unique character count
    sorted_indices = np.argsort(unique_char_count)
    sorted_arr = arr[sorted_indices]

    # Convert the sorted NumPy array back to a list
    sorted_list = sorted_arr.tolist()

    return sorted_list

# Example usage
test_list = ['gfg', 'best', 'for', 'geeks']
print(sort_list_unique_chars(test_list))

Output:

[‘gfg’, ‘for’, ‘best’, ‘geeks’]

Time Complexity: O(nlogn)

Auxiliary Space: O(n)

Method 6: Using QuickSort and count of unique characters

1. Define a helper function called count_unique_chars that takes a string and returns the count of unique characters in the string using the set and len functions.
2. Define a partition function called partition that takes a list, a low index, and a high index, and returns the index of the pivot element after partitioning the list around the pivot element. The pivot element is the first element in the list (arr[low]). We set two pointers i and j initially pointing to the low and high indices, respectively. We then iterate through the list using the two pointers, swapping elements if necessary, until the pointers cross each other. Finally, we swap the pivot element with the element at index j and return j.
3. Define a quicksort function called quicksort that takes a list, a low index, and a high index. If the low index is less than the high index, we partition the list around a pivot index using the partition function and recursively call quicksort on the two resulting sublists to sort them.
4. Initialize a test list of strings called test_list.
5. Call quicksort on test_list, passing in 0 as the low index and len(test_list)-1 as the high index, to sort the list based on count of unique characters.
6. Print the sorted list using the print function.

Python3

# Python3 code to demonstrate working of
# Sort Strings by Unique characters
# Using QuickSort and count of unique characters

# helper function to return count of unique characters in a string
def count_unique_chars(s):
    return len(set(s))

# partition function for QuickSort
def partition(arr, low, high):
    pivot = arr[low]
    i = low + 1
    j = high
    while True:
        while i <= j and count_unique_chars(arr[i]) <= count_unique_chars(pivot):
            i += 1
        while i <= j and count_unique_chars(arr[j]) >= count_unique_chars(pivot):
            j -= 1
        if i <= j:
            arr[i], arr[j] = arr[j], arr[i]
        else:
            break
    arr[low], arr[j] = arr[j], arr[low]
    return j

# QuickSort algorithm to sort list based on count of unique characters
def quicksort(arr, low, high):
    if low < high:
        pivot_index = partition(arr, low, high)
        quicksort(arr, low, pivot_index - 1)
        quicksort(arr, pivot_index + 1, high)

# initializing list
test_list = ['gfg', 'best', 'for', 'geeks']

# use QuickSort algorithm to sort list based on count of unique characters
quicksort(test_list, 0, len(test_list) - 1)

# print sorted list
print(test_list)

Output

['gfg', 'for', 'geeks', 'best']

The time complexity of the partition function is O(n), and the quicksort function calls the partition function recursively on each sub-list, so the time complexity of quicksort is O(n log n) in the average case and O(n^2) in the worst case.

Auxiliary Space:
The partition function uses a constant amount of auxiliary space, and quicksort uses O(log n) auxiliary space in the average case and O(n) in the worst case due to the recursion depth. Therefore, the overall space complexity of this method is O(log n) in the average case and O(n) in the worst case.