Tuesday, 19 Oct 2021

# Print all array elements having frequencies equal to powers of K in ascending order

Print all array elements having frequencies equal to powers of K in ascending order
Given an array arr[] consisting of N integers and a positive integer K, the task is to find the array elements having frequencies in the power of K i.e., K1, K2, K3, and so on.
Examples:

Input: arr[] = {1, 3, 2, 1, 2, 2, 2, 3, 3, 4}, K = 2Output: 1 2Explanation:The frequency of 1 is 2, that can be represented as the power of K( = 2), i.e., 21.The frequency of 2 is 4, that can be represented as the power of K( = 2), i.e., 22.
Input: arr[] = {6, 1, 3, 1, 2, 2, 1}, K = 2Output: 2 3 6

Naive Approach: The simplest approach is to count the frequencies of each array element and if the frequency of any element is a perfect power of K, then print that element. Otherwise, check for the next element.Time Complexity: O(N2)Auxiliary Space: O(1)
Efficient Approach: The above approach can also be optimized by using Hashing for storing the frequency of arrays elements in a HashMap and then check for the required conditions. Follow the steps below to solve the given problem:
Traverse the given array arr[] and store the frequency of each array element in a Map, say M.
Now, traverse the map and perform the following steps:
Store the frequency of each value in the map in a variable, say F.
If the value of (log F)/(log K) and K(log F)/(log K) are the same, then the current element has the frequency as the perfect power of K. Therefore, print the current element.

Below is the implementation of the above approach:

C++

#include
using namespace std;

void countFrequency(int arr[], int N,
int K)
{

unordered_map freq;

for (int i = 0; i < N; i++) {                              freq[arr[i]]++;     }             for (auto i : freq) {                              int lg = log(i.second) / log(K);                     int a = pow(K, lg);                     if (a == i.second) {                             cout