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

The brute force approach for Find Longest Special Substring That Occurs Thrice I 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. For each substring, we determine if it is special and count its occurrences in the string. This is straightforward but inefficient for larger 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 I?

Find Longest Special Substring That Occurs Thrice I 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 I?

Clarify the definition of special substrings with the interviewer. Discuss your thought process while coding to show your understanding. Consider edge cases, such as strings without any special substrings.

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

Not checking for special substrings correctly (e.g., using incorrect conditions). Overlooking the need to count occurrences properly.

#2981

Find Longest Special Substring That Occurs Thrice I

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)

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Intuition

Time O(n)Space O(n)

The optimal solution leverages the fact that special substrings consist of repeated characters. We can count the lengths of consecutive characters and then check how many times each length appears. This reduces the number of checks significantly.

⚙️

Algorithm

4 steps

1Step 1: Traverse the string and count consecutive characters, storing their lengths in a list.
2Step 2: Use a dictionary to count how many times each length appears.
3Step 3: Iterate through the dictionary to find the maximum length that appears at least three times.
4Step 4: Return the maximum length found, or -1 if none exists.

solution.py17 lines

1def longest_special_substring(s):
2    count = {}
3    n = len(s)
4    i = 0
5    while i < n:
6        char = s[i]
7        length = 0
8        while i < n and s[i] == char:
9            length += 1
10            i += 1
11        if length > 0:
12            count[length] = count.get(length, 0) + 1
13    max_length = -1
14    for length, freq in count.items():
15        if freq >= 3:
16            max_length = max(max_length, length)
17    return max_length

ℹ

Complexity note: The time complexity is O(n) because we traverse the string once to count lengths of special substrings. The space complexity is O(n) due to the dictionary storing counts of lengths.

1Special substrings are formed by consecutive identical characters.
2Using a dictionary to count occurrences can significantly reduce checks.

Solutions and explanations are original Tejav content. Problem titles © LeetCode — use the LeetCode button above for the full problem statement.