Print all characters of string whose frequency is a power of K

Given string str of size N, the task is to print the characters of string whose frequency is a power of K in a lexicographically sorted order.

Examples:

Input: str = "aaacbb" K = 2
Output: bbc
Explanation: Frequency of a is 3 which is not the power of 2. Frequency of c is 1 and frequency of b is 2 which are the power of 2. 

Input: str = "geeksgeekgeeks" K = 3
Output: eeeeeegggkkk

Naive Approach: The idea is to count frequency for every alphabet of the string, if the frequency is the power of K then add it to a new string. Sort the string and print it.

Time Complexity: O(N²)
Auxiliary Space: O(N)

Efficient Approach: The idea is to use Hashing. Below are the steps:

• Traverse the string and store the frequency of each alphabet in a Map
• Traverse the map and print the alphabets whose frequency is the power of K

Below is the implementation of the above approach:

b
b
c

Time Complexity: O(N * log N)
Auxiliary Space: O(N)