Given an array, find the least frequent element in it. If there are multiple elements that appear least number of times, print any one of them.
Examples :
Input : arr[] = {1, 3, 2, 1, 2, 2, 3, 1}
Output : 3
3 appears minimum number of times in given
array.
Input : arr[] = {10, 20, 30}
Output : 10 or 20 or 30
Method 1:
A simple solution is to run two loops. The outer loop picks all elements one by one. The inner loop finds frequency of the picked element and compares with the minimum so far. Time complexity of this solution is O(n2)
A better solution is to do sorting. We first sort the array, then linearly traverse the array.
Python3
# Python 3 program to find the least# frequent element in an array.def leastFrequent(arr, n): # Sort the array arr.sort() # find the min frequency using # linear traversal min_count = n + 1 res = -1 curr_count = 1 for i in range(1, n): if (arr[i] == arr[i - 1]): curr_count = curr_count + 1 else: if (curr_count < min_count): min_count = curr_count res = arr[i - 1] curr_count = 1 # If last element is least frequent if (curr_count < min_count): min_count = curr_count res = arr[n - 1] return res# Driver programarr = [1, 3, 2, 1, 2, 2, 3, 1]n = len(arr)print(leastFrequent(arr, n)) |
3
Time Complexity : O(n Log n)
Auxiliary Space : O(1)
Method 2:
An efficient solution is to use hashing. We create a hash table and store elements and their frequency counts as key value pairs. Finally we traverse the hash table and print the key with minimum value.
Python3
# Python3 program to find the most # frequent element in an array.import math as mtdef leastFrequent(arr, n): # Insert all elements in Hash. Hash = dict() for i in range(n): if arr[i] in Hash.keys(): Hash[arr[i]] += 1 else: Hash[arr[i]] = 1 # find the max frequency min_count = n + 1 res = -1 for i in Hash: if (min_count >= Hash[i]): res = i min_count = Hash[i] return res# Driver Codearr = [1, 3, 2, 1, 2, 2, 3, 1] n = len(arr)print(leastFrequent(arr, n)) |
3
Time Complexity : O(n)
Auxiliary Space : O(n)
Method 3: Using Counter() Function
Python3
# Python3 program to find the most# frequent element in an array.import math as mtfrom collections import Counterdef leastFrequent(arr, n): # Getting frequency of all elements Hash = Counter(arr) # find the max frequency min_count = n + 1 res = -1 for i in Hash: if (min_count >= Hash[i]): res = i min_count = Hash[i] return res# Driver Codearr = [1, 3, 2, 1, 2, 2, 3, 1]n = len(arr)print(leastFrequent(arr, n)) |
3
Please refer complete article on Least frequent element in an array for more details!
