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Count the Number of Special Characters I

Difficulty: Easy

Problem Description

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

Key Insights

  • A special character must have both its lowercase and uppercase forms present in the string.
  • Using a hash table (or set) can efficiently track the occurrence of characters in both cases.
  • The characters can be processed in a single pass through the string, ensuring O(n) time complexity.

Space and Time Complexity

Time Complexity: O(n), where n is the length of the input string word.
Space Complexity: O(1), since we are using a fixed number of additional data structures (26 letters).

Solution

To solve this problem, we can utilize two sets to track the occurrence of lowercase and uppercase letters separately. As we iterate through the string, we will add each character to its respective set. Finally, we will compare the two sets to count how many characters appear in both cases.

Code Solutions

def count_special_characters(word):
    lower_set = set()
    upper_set = set()
    
    for char in word:
        if char.islower():
            lower_set.add(char)
        elif char.isupper():
            upper_set.add(char)
    
    special_count = 0
    for char in lower_set:
        if char.upper() in upper_set:
            special_count += 1
            
    return special_count
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