Tracxn F2F interview

You are given:

• A string s of lowercase English letters.
• An integer k, representing the required length of each substring.
• A Map<Character, Integer> charCount, where each entry (ch, freq) specifies that character ch must appear at least freq times in a valid substring.

Your task:
Return a list of all unique substrings of length k from the string s such that:

• Each character in charCount appears in the substring at least the number of times specified.
• Substrings are considered unique, so repeated substrings should only appear once in the result.

without using substring and built in functions of string

Input:
s = "abacad"
k = 3
charCount = { 'a': 2, 'b': 1 }

Output:
["aba"]

Explanation:
- "aba" has two 'a's and one 'b'
- Other substrings of length 3: "bac", "aca", "cad" don't meet the character frequency criteria.