3120. Count the Number of Special Characters I

3120. Count the Number of Special Characters I

Difficulty: Easy

Topics: Mid Level, Hash Table, String, Weekly Contest 394

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.

Hint:

1. The constraints are small. For all 52 characters, check if they are present in word.

Solution:

The solution determines how many English letters appear in both lowercase and uppercase forms within a given string. It uses two boolean arrays to track the presence of each letter (a–z) in lowercase and uppercase separately, then counts the letters that appear in both arrays.

Approach:

• Track lowercase and uppercase letters separately using two fixed-size boolean arrays (size 26).
• Iterate through the string and mark the appropriate array based on whether the character is lowercase or uppercase.
• Count matches by checking both arrays for the same letter index.

Let's implement this solution in PHP: 3120. Count the Number of Special Characters I

<?php
/**
 * @param String $word
 * @return Integer
 */
function numberOfSpecialChars(string $word): int
{
    ...
    ...
    ...
    /**
     * go to ./solution.php
     */
}

// Test cases
echo numberOfSpecialChars("aaAbcBC") . "\n";            // Output: 3
echo numberOfSpecialChars("abc") . "\n";                // Output: 0
echo numberOfSpecialChars("abBCab") . "\n";             // Output: 1
?>

Explanation:

• Initialize two arrays $lower and $upper of size 26, all set to false. Index 0 represents 'a'/'A', index 1 represents 'b'/'B', etc.
• Loop through each character in the string:
  • If the character is lowercase, mark $lower[index] = true.
  • If the character is uppercase, mark $upper[index] = true.
• After processing the string, loop through indexes 0 to 25:
  • If both $lower[i] and $upper[i] are true, increment the count.
• Return the total count of special characters.

Complexity

• Time Complexity: O(n), One pass through the string (n = length of word) and one pass through the 26 letters → O(n + 26) → O(n).
• Space Complexity: O(1), Two fixed-size arrays of 26 booleans → constant space.

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