A competitive anagram problem involving permutations could be framed as follows: Problem Statement: Anagram Substring Search with Constraints
Given a string txt and a string pat, your task is to find all occurrences of pat and its permutations (anagrams) in txt, but with the following constraints:
Example
Input:
txt = "BACDGABCDA"
pat = "ABCD"
L = 4
Output:
[0, 5, 6]
Explanation: The valid substrings that are permutations of pat and within the length constraint are "BACD" at index 0, "ABCD" at index 5, and "BCDA" at index 6.
Implementation Considerations
To solve this problem efficiently, you can use a sliding window approach combined with character frequency counting. Here’s a high-level outline of the algorithm:
Character Count: Create a frequency count of characters in pat.
Sliding Window: Use a sliding window of size equal to the length of pat (or L, whichever is smaller) to traverse txt.
Frequency Comparison: For each window, compare the character frequencies with those of pat. If they match, record the starting index.
Edge Cases: Handle cases where pat is longer than L or contains characters not present in txt.
This problem not only tests your understanding of permutations and anagrams but also challenges your ability to implement efficient algorithms using data structures like hash maps for frequency counting. Additional Challenge
To increase the complexity, you could also require that the output indices be sorted in descending order or that the function returns the actual substrings instead of just the indices.
Can you provide a Python code example implementing this algorithm?