What is the brute force solution for Find Longest Special Substring That Occurs Thrice II?

The brute force approach for Find Longest Special Substring That Occurs Thrice II is Brute Force, with O(n²) time complexity and O(1) space complexity. The brute force approach checks all possible substrings of the input string to see if they are special and count their occurrences. This method is straightforward but inefficient for longer strings. The optimal Optimal Solution approach improves this to O(n).

What algorithm or data structure is used to solve Find Longest Special Substring That Occurs Thrice II?

Find Longest Special Substring That Occurs Thrice II uses the following concepts: Hash Table, String, Binary Search, Sliding Window, Counting. The recommended patterns to study are: Hash Map, Array.

What are the interview tips for Find Longest Special Substring That Occurs Thrice II?

Start with a brute-force approach to demonstrate understanding, but be ready to optimize. Explain your thought process clearly, especially when transitioning from brute-force to optimal solutions. Practice identifying patterns in string problems, as they often lead to more efficient solutions.

What are common mistakes when solving Find Longest Special Substring That Occurs Thrice II?

Not considering the case where no special substring occurs thrice. Overlooking the need to check for the maximum length after counting.

#2982

Find Longest Special Substring That Occurs Thrice II

Medium

Hash TableStringBinary SearchSliding WindowCountingHash MapArray

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Approaches

Brute ForceOptimal

Complexity Comparison

	Brute Force	Optimal Solution★
Time	O(n²)	O(n)
Space	O(1)	O(n)

💡

Intuition

Time O(n)Space O(n)

This approach uses a single pass to determine the lengths of special substrings and a hash map to count their occurrences efficiently. This reduces the time complexity significantly.

⚙️

Algorithm

3 steps

1Step 1: Traverse the string to find lengths of contiguous special substrings and store them in an array.
2Step 2: Use a hash map to count how many times each special substring length appears.
3Step 3: Check the hash map for lengths that appear at least three times and return the maximum length found.

solution.py27 lines

1# Full working Python code
2
3def longest_special_substring(s):
4    from collections import defaultdict
5    n = len(s)
6    lengths = []
7    count_map = defaultdict(int)
8
9    # Step 1: Find lengths of special substrings
10    current_length = 1
11    for i in range(1, n):
12        if s[i] == s[i - 1]:
13            current_length += 1
14        else:
15            lengths.append(current_length)
16            count_map[current_length] += 1
17            current_length = 1
18    lengths.append(current_length)
19    count_map[current_length] += 1
20
21    # Step 2: Find the maximum length that occurs at least thrice
22    max_length = -1
23    for length, count in count_map.items():
24        if count >= 3:
25            max_length = max(max_length, length)
26
27    return max_length

ℹ

Complexity note: The time complexity is O(n) because we only traverse the string once to gather lengths of special substrings, and the space complexity is O(n) due to the hash map used for counting.

1Special substrings are contiguous sequences of the same character.
2Using a hash map allows for efficient counting of substring lengths.

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