What is the brute force solution for Count Unique Characters of All Substrings of a Given String?

The brute force approach for Count Unique Characters of All Substrings of a Given String is Brute Force, with O(n²) time complexity and O(1) space complexity. The brute force approach generates all possible substrings of the given string and counts the unique characters in each substring. While this method is straightforward, it can be inefficient for larger strings due to the exponential number of substrings. The optimal Optimal Solution approach improves this to O(n).

What algorithm or data structure is used to solve Count Unique Characters of All Substrings of a Given String?

Count Unique Characters of All Substrings of a Given String uses the following concepts: Hash Table, String, Dynamic Programming. The recommended patterns to study are: Hash Map, Array.

What are the interview tips for Count Unique Characters of All Substrings of a Given String?

Explain your thought process clearly while coding, especially when transitioning from brute force to optimal solutions. Be prepared to discuss the trade-offs between different approaches. Practice explaining your code and logic as if teaching someone else.

What are common mistakes when solving Count Unique Characters of All Substrings of a Given String?

Failing to account for the last occurrence of characters, leading to incorrect counts. Not recognizing the need to sum contributions rather than counting unique characters directly.

#828

Count Unique Characters of All Substrings of a Given String

Hard

Hash TableStringDynamic ProgrammingHash MapArray

LeetCode ↗

Approaches

Brute ForceOptimal

Complexity Comparison

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

💡

Intuition

Time O(n)Space O(1)

The optimal approach leverages the idea of tracking the last occurrence of each character to efficiently calculate the contribution of each character to the unique character count in substrings. This avoids the need to generate all substrings explicitly.

⚙️

Algorithm

3 steps

1Step 1: Initialize a last occurrence array to store the last index of each character.
2Step 2: Iterate through the string, maintaining a count of unique characters contributed by each character.
3Step 3: For each character, calculate its contribution based on its last occurrence and update the total count.

solution.py12 lines

1# Full working Python code
2
3def countUniqueChars(s):
4    last_occurrence = [-1] * 26
5    total_count = 0
6    n = len(s)
7    for i in range(n):
8        char_index = ord(s[i]) - ord('A')
9        last_index = last_occurrence[char_index]
10        total_count += (i - last_index) * (n - i)
11        last_occurrence[char_index] = i
12    return total_count

ℹ

Complexity note: The time complexity is O(n) because we make a single pass through the string, and each character's contribution is calculated in constant time. The space complexity is O(1) since the size of the last occurrence array is fixed at 26 (for each uppercase letter).

1Each character's contribution to unique counts can be calculated based on its position and last occurrence.
2Understanding how to efficiently track character positions can significantly reduce computation time.

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