#3121

Medium

LeetCode Problem Workspace

Count the Number of Special Characters II

Count the Number of Special Characters II is a medium difficulty string and hash table problem that involves identifying special characters based on case conditions.

hash table string

Problem Statement

Given a string word, a letter c is called special if it appears both in lowercase and uppercase in the string, and every lowercase occurrence of c appears before its first uppercase occurrence. The task is to count the number of such special letters in the string.

For example, in the string aaAbcBC, the special characters are 'a', 'b', and 'c' since they appear in both lowercase and uppercase with the correct ordering of occurrences. If no such characters exist, the result should be 0.

Examples

Example 1

Input: word = "aaAbcBC"

Output: 3

The special characters are 'a' , 'b' , and 'c' .

Example 2

Input: word = "abc"

Output: 0

There are no special characters in word .

Example 3

Input: word = "AbBCab"

Output: 0

There are no special characters in word .

Constraints

1 <= word.length <= 2 * 105
word consists of only lowercase and uppercase English letters.

Solution Approach

Hash Table to Track Occurrences

Use a hash table to track both the first occurrence of a letter in uppercase and the last occurrence in lowercase. This allows quick checks for whether a character is special.

Two Passes Through the String

First, iterate through the string to store positions of lowercase and uppercase letters. Then, in a second pass, check each character to verify if it satisfies the special character condition.

Efficient Search and Count

Once the data is stored, efficiently count the special characters by checking if both lowercase and uppercase occurrences are recorded in the hash table and that the lowercase version comes before the uppercase.

Complexity Analysis

Metric	Value
Time	Depends on the final approach
Space	Depends on the final approach

The time complexity of the solution depends on the approach. With the two passes and hash table usage, it can be O(n) where n is the length of the string. The space complexity is also O(n) due to the hash table storing character positions.

What Interviewers Usually Probe

Look for an efficient solution that minimizes both time and space complexity using hash tables.
Ensure the candidate recognizes the need for two passes or a similar approach to process both lowercase and uppercase occurrences.
Evaluate whether the candidate can effectively balance clarity and optimization in their approach.

Common Pitfalls or Variants

Common pitfalls

Failing to account for the proper order of lowercase and uppercase occurrences can lead to incorrect results.
Using an inefficient solution that does not utilize a hash table to track positions properly.
Not considering edge cases, such as strings with no special characters or strings where characters appear only in one case.

Follow-up variants

Extend the problem to include numbers or other special characters.
Add a constraint to allow only specific subsets of characters (e.g., only letters A-Z, or only lowercase).
Consider optimizing space usage by using a more space-efficient data structure instead of a hash table.

FAQ

What is the core pattern of the problem 'Count the Number of Special Characters II'?

The core pattern involves using hash tables to efficiently track the first occurrence of uppercase and last occurrence of lowercase letters, ensuring the correct ordering.

How do you handle case-sensitive letter counting in this problem?

By using a hash table to store the positions of both lowercase and uppercase occurrences, you can quickly check the order and count special characters.

Can this problem be solved with just string manipulation?

While string manipulation is useful, using a hash table provides a much more efficient solution to track character occurrences and ensure proper ordering.

What is the time complexity of solving the problem?

The time complexity is O(n), where n is the length of the string, as you perform two passes over the string and use a hash table for constant-time lookups.

Are there any edge cases to consider in this problem?

Yes, edge cases include strings with no special characters, strings where characters appear only in one case, and very large strings up to the given length constraint.

terminal

Solution

Solution 1: Hash Table or Array

We define two hash tables or arrays `first` and `last` to store the positions where each letter first appears and last appears respectively.

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class Solution:
    def numberOfSpecialChars(self, word: str) -> int:
        first, last = {}, {}
        for i, c in enumerate(word):
            if c not in first:
                first[c] = i
            last[c] = i
        return sum(
            a in last and b in first and last[a] < first[b]
            for a, b in zip(ascii_lowercase, ascii_uppercase)
        )

Continue Practicing

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