#3120

Easy

LeetCode Problem Workspace

Count the Number of Special Characters I

Count the number of special characters in a given string using hash tables and string manipulation.

hash table string

Problem Statement

You are given a string word. A letter is called special if it appears both in lowercase and uppercase in the word.

Return the number of special letters in word.

Examples

Example 1

Input: word = "aaAbcBC"

Output: 3

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

Example 2

Input: word = "abc"

Output: 0

No character in word appears in uppercase.

Example 3

Input: word = "abBCab"

Output: 1

The only special character in word is 'b' .

Constraints

1 <= word.length <= 50
word consists of only lowercase and uppercase English letters.

Solution Approach

Using a Hash Table

One efficient solution is to use a hash table (or dictionary) to store the count of each character. Traverse the string and update the hash table. Finally, check for any letter that has both its lowercase and uppercase occurrences.

Brute Force with Two Passes

Another method involves iterating through the string twice, first to find all lowercase letters and then to check for their uppercase counterparts. This solution may not be optimal but works for small strings.

Set-based Approach

A third approach involves using two sets, one for lowercase letters and one for uppercase letters. Iterate through the string and populate the sets. The special letters are the intersection of these sets.

Complexity Analysis

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

Time complexity depends on the approach. The hash table approach is O(n), where n is the length of the string. The brute force method is O(n^2), and the set-based approach also works in O(n), making it efficient for the given constraints. Space complexity is O(1) for the hash table or sets, as the size is limited to the 26 characters in each case.

What Interviewers Usually Probe

Focus on solving the problem efficiently within the given constraints.
A correct approach with a hash table will demonstrate the ability to optimize with space and time.
Look for the candidate's ability to handle edge cases like strings with no uppercase characters.

Common Pitfalls or Variants

Common pitfalls

Over-complicating the solution with unnecessary passes through the string.
Ignoring edge cases, such as when there are no special characters or when the string is very short.
Using inefficient data structures that may lead to excessive time complexity.

Follow-up variants

Change the input string to include digits or special characters and see if the solution adapts.
Increase the string length to test the performance of the algorithm.
Test the solution with uppercase and lowercase characters mixed in various sequences.

FAQ

What is the best approach to solving the Count the Number of Special Characters I problem?

The hash table approach is the most efficient, allowing you to track character occurrences and check for special letters in one pass.

How does the hash table approach work in this problem?

By storing the occurrence of each character, you can efficiently check if a character appears both in lowercase and uppercase in a single pass.

What are common pitfalls in solving this problem?

A common mistake is over-complicating the problem with unnecessary iterations or failing to handle edge cases like empty strings.

How does GhostInterview help with this problem?

GhostInterview offers insights into optimal solutions and helps you avoid common pitfalls, improving both accuracy and efficiency.

Can the solution be optimized further for larger inputs?

The hash table approach is already optimal in terms of time complexity for this problem, but you can experiment with different data structures for space efficiency.

terminal

Solution

Solution 1: Hash Table or Array

We use a hash table or array $s$ to record the characters that appear in the string $word$. Then we traverse the 26 letters. If both the lowercase and uppercase letters appear in $s$, the count of special characters is incremented by one.

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class Solution:
    def numberOfSpecialChars(self, word: str) -> int:
        s = set(word)
        return sum(a in s and b in s for a, b in zip(ascii_lowercase, ascii_uppercase))

Continue Practicing

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