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Count the Number of Special Characters I

Easy

Asked by:

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Topics:

Strings

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.

Example 1:

Input: word = "aaAbcBC"

Output: 3

Explanation:

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

Example 2:

Input: word = "abc"

Output: 0

Explanation:

No character in word appears in uppercase.

Example 3:

Input: word = "abBCab"

Output: 1

Explanation:

The only special character in word is 'b'.

Constraints:

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

Try coding it in LeetCode

Count the Number of Special Characters I

Easy

Asked by:

12 views

Topics:

Strings

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.

Example 1:

Input: word = "aaAbcBC"

Output: 3

Explanation:

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

Example 2:

Input: word = "abc"

Output: 0

Explanation:

No character in word appears in uppercase.

Example 3:

Input: word = "abBCab"

Output: 1

Explanation:

The only special character in word is 'b'.

Constraints:

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

Try coding it in LeetCode

Solution

Clarifying Questions

When you get asked this question in a real-life environment, it will often be ambiguous (especially at FAANG). Make sure to ask these questions in that case:

What characters are considered "special"? Can you provide a definitive list or examples beyond alphanumeric characters?
What is the expected data type of the input? Is it a string, a character array, or something else?
Are there any limitations on the length of the input string/array? What is the maximum possible length I should consider?
Should I expect null or empty input? If so, what should the function return in those cases?
Does case sensitivity matter when determining if a character is "special"? Should uppercase and lowercase letters be treated the same?

Brute Force Solution

Approach

The goal is to count how many special characters are in a given piece of text. The brute force approach is very straightforward: we will examine each character in the text, one at a time. For each character, we'll check if it is a special character.

Here's how the algorithm would work step-by-step:

Start at the very beginning of the text.
Look at the first character.
Check if that character is a special character (like symbols, punctuation, or anything that isn't a regular letter or number).
If it is special, increase the count of special characters by one.
Move on to the next character in the text.
Repeat the checking and counting process for every single character in the entire text.
Once you have checked all characters, the final count is the number of special characters in the text.

Code Implementation

def count_special_characters_brute_force(text):
    special_character_count = 0
    
    for character in text:
        # Check if the character is not alphanumeric
        if not character.isalnum():

            # Increment the counter because it's a special character
            special_character_count += 1

    return special_character_count

Big(O) Analysis

Time Complexity

O(n) – The algorithm iterates through each character in the input text once. For each of the n characters in the text, a constant-time check is performed to determine if it is a special character. Since the number of operations is directly proportional to the length of the input text, the time complexity is O(n).

Space Complexity

O(1) – The provided algorithm iterates through the input text character by character but does not create any auxiliary data structures like arrays, hash maps, or lists to store intermediate results or track visited characters. It only uses a single counter variable to keep track of the number of special characters found. Thus, the auxiliary space used remains constant, independent of the size of the input text (N), and hence the space complexity is O(1).

Optimal Solution

Approach

The goal is to efficiently count characters that are not letters or numbers. We can do this by looking at each character in the input and checking if it's special. This avoids complicated comparisons and directly addresses the question.

Here's how the algorithm would work step-by-step:

Take the given input, which is a string of characters.
Examine each character individually, one at a time.
For each character, check if it is a letter (a to z, or A to Z) or a number (0 to 9).
If the character is NOT a letter or a number, then it must be a special character. Increase the count.
After checking every character in the input, the final count represents the total number of special characters.

Code Implementation

def count_the_number_of_special_characters_i(input_string):
    special_character_count = 0

    for character in input_string:
        # Check if the character is an alphabet or a number
        if not ('a' <= character <= 'z' or 'A' <= character <= 'Z' or '0' <= character <= '9'):

            # Increment the counter because it's a special character
            special_character_count += 1

    return special_character_count

def is_special_character(char):
    # Checking if a char is a letter or number requires a helper function
    if ('a' <= char <= 'z' or 'A' <= char <= 'Z' or '0' <= char <= '9'):
        return False
    else:
        return True

def count_special_characters_optimized(input_string):
    number_of_special_characters = 0

    for char_from_input in input_string:
        # is_special_character improves code readability
        if is_special_character(char_from_input):

            number_of_special_characters += 1

    return number_of_special_characters

def count_special_characters_comp(input_string):
    # Filter the input string to exclude alphanumeric characters
    special_characters = [char for char in input_string if not char.isalnum()]

    #The length of the filtered string equals the number of special chars
    return len(special_characters)

def count_special_characters_unicode(input_string):
    count = 0
    for character in input_string:
        # Use unicode categories to identify special characters.
        if not ('Ll' <= character <= 'Lu' or 'Nd' <= character <= 'Nd'):

            count += 1

    return count

Big(O) Analysis

Time Complexity

O(n) – The algorithm iterates through each character of the input string once. For each character, a constant amount of work is performed to check if it's a letter or a number. Therefore, the time complexity is directly proportional to the length of the string, n, resulting in O(n) time complexity.

Space Complexity

O(1) – The described algorithm iterates through the input string character by character and maintains a single counter variable to track the number of special characters. No additional data structures like arrays, lists, or hash maps are created to store intermediate results or track visited characters. The memory used by the counter variable remains constant irrespective of the input string's length, denoted as N. Therefore, the auxiliary space complexity is O(1).

Edge Cases

Case	How to Handle
Null or empty string input	Return 0 since there are no characters to examine.
String containing only special characters	Return the length of the string, as all characters are special.
String containing no special characters	Return 0 since no characters meet the special character criteria.
String with maximum allowed length (system dependent)	Ensure the algorithm has acceptable time complexity for long strings (e.g., O(n)).
String contains Unicode characters including special characters	Ensure the special character check correctly handles Unicode characters using appropriate character encoding.
String contains extremely long sequence of consecutive special characters	The count should increment for each consecutive special character without overflow issues.
Special characters are defined as a very large set	Use an efficient data structure (e.g., a set) to check for special characters to avoid O(n) lookup per character.
Integer overflow when counting a very long sequence of special characters	Use a data type that can accommodate large counts (e.g., long) or add overflow checking.

Null or empty string input

How to Handle:

Return 0 since there are no characters to examine.

String containing only special characters

How to Handle:

Return the length of the string, as all characters are special.

String containing no special characters

How to Handle:

Return 0 since no characters meet the special character criteria.

String with maximum allowed length (system dependent)

How to Handle:

Ensure the algorithm has acceptable time complexity for long strings (e.g., O(n)).

String contains Unicode characters including special characters

How to Handle:

Ensure the special character check correctly handles Unicode characters using appropriate character encoding.

String contains extremely long sequence of consecutive special characters

How to Handle:

The count should increment for each consecutive special character without overflow issues.

Special characters are defined as a very large set

How to Handle:

Use an efficient data structure (e.g., a set) to check for special characters to avoid O(n) lookup per character.

Integer overflow when counting a very long sequence of special characters

How to Handle:

Use a data type that can accommodate large counts (e.g., long) or add overflow checking.

Solution

Clarifying Questions

When you get asked this question in a real-life environment, it will often be ambiguous (especially at FAANG). Make sure to ask these questions in that case:

What characters are considered "special"? Can you provide a definitive list or examples beyond alphanumeric characters?
What is the expected data type of the input? Is it a string, a character array, or something else?
Are there any limitations on the length of the input string/array? What is the maximum possible length I should consider?
Should I expect null or empty input? If so, what should the function return in those cases?
Does case sensitivity matter when determining if a character is "special"? Should uppercase and lowercase letters be treated the same?

Brute Force Solution

Approach

Here's how the algorithm would work step-by-step:

Start at the very beginning of the text.
Look at the first character.
Check if that character is a special character (like symbols, punctuation, or anything that isn't a regular letter or number).
If it is special, increase the count of special characters by one.
Move on to the next character in the text.
Repeat the checking and counting process for every single character in the entire text.
Once you have checked all characters, the final count is the number of special characters in the text.

Code Implementation

def count_special_characters_brute_force(text):
    special_character_count = 0
    
    for character in text:
        # Check if the character is not alphanumeric
        if not character.isalnum():

            # Increment the counter because it's a special character
            special_character_count += 1

    return special_character_count

Big(O) Analysis

Time Complexity

Space Complexity

Optimal Solution

Approach

Here's how the algorithm would work step-by-step:

Take the given input, which is a string of characters.
Examine each character individually, one at a time.
For each character, check if it is a letter (a to z, or A to Z) or a number (0 to 9).
If the character is NOT a letter or a number, then it must be a special character. Increase the count.
After checking every character in the input, the final count represents the total number of special characters.

Code Implementation

def count_the_number_of_special_characters_i(input_string):
    special_character_count = 0

    for character in input_string:
        # Check if the character is an alphabet or a number
        if not ('a' <= character <= 'z' or 'A' <= character <= 'Z' or '0' <= character <= '9'):

            # Increment the counter because it's a special character
            special_character_count += 1

    return special_character_count

def is_special_character(char):
    # Checking if a char is a letter or number requires a helper function
    if ('a' <= char <= 'z' or 'A' <= char <= 'Z' or '0' <= char <= '9'):
        return False
    else:
        return True

def count_special_characters_optimized(input_string):
    number_of_special_characters = 0

    for char_from_input in input_string:
        # is_special_character improves code readability
        if is_special_character(char_from_input):

            number_of_special_characters += 1

    return number_of_special_characters

def count_special_characters_comp(input_string):
    # Filter the input string to exclude alphanumeric characters
    special_characters = [char for char in input_string if not char.isalnum()]

    #The length of the filtered string equals the number of special chars
    return len(special_characters)

def count_special_characters_unicode(input_string):
    count = 0
    for character in input_string:
        # Use unicode categories to identify special characters.
        if not ('Ll' <= character <= 'Lu' or 'Nd' <= character <= 'Nd'):

            count += 1

    return count

Big(O) Analysis

Time Complexity

Space Complexity

Edge Cases

Case	How to Handle
Null or empty string input	Return 0 since there are no characters to examine.
String containing only special characters	Return the length of the string, as all characters are special.
String containing no special characters	Return 0 since no characters meet the special character criteria.
String with maximum allowed length (system dependent)	Ensure the algorithm has acceptable time complexity for long strings (e.g., O(n)).
String contains Unicode characters including special characters	Ensure the special character check correctly handles Unicode characters using appropriate character encoding.
String contains extremely long sequence of consecutive special characters	The count should increment for each consecutive special character without overflow issues.
Special characters are defined as a very large set	Use an efficient data structure (e.g., a set) to check for special characters to avoid O(n) lookup per character.
Integer overflow when counting a very long sequence of special characters	Use a data type that can accommodate large counts (e.g., long) or add overflow checking.

Null or empty string input

How to Handle:

Return 0 since there are no characters to examine.

String containing only special characters

How to Handle:

Return the length of the string, as all characters are special.

String containing no special characters

How to Handle:

Return 0 since no characters meet the special character criteria.

String with maximum allowed length (system dependent)

How to Handle:

Ensure the algorithm has acceptable time complexity for long strings (e.g., O(n)).

String contains Unicode characters including special characters

How to Handle:

Ensure the special character check correctly handles Unicode characters using appropriate character encoding.

String contains extremely long sequence of consecutive special characters

How to Handle:

The count should increment for each consecutive special character without overflow issues.

Special characters are defined as a very large set

How to Handle:

Use an efficient data structure (e.g., a set) to check for special characters to avoid O(n) lookup per character.

Integer overflow when counting a very long sequence of special characters

How to Handle:

Use a data type that can accommodate large counts (e.g., long) or add overflow checking.