Count the Number of Special Characters I - Practice Coding Problems

Count the Number of Special Characters I - Problem

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.

Input & Output

Example 1 — Mixed Case Letters

$ Input: word = "aaAbcBC"

› Output: 3

💡 Note: Letters 'a', 'b', and 'c' each appear in both lowercase and uppercase forms

Example 2 — Only Lowercase

$ Input: word = "abc"

› Output: 0

💡 Note: All letters are lowercase only, no uppercase versions present

Example 3 — Partial Match

$ Input: word = "abBcab"

› Output: 1

💡 Note: Only letter 'b' appears in both cases (lowercase 'b' and uppercase 'B')

Constraints

1 ≤ word.length ≤ 1000
word consists of only lowercase and uppercase English letters.

Visualization

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Understanding the Visualization

Input String

String with mixed case letters

Identify Pairs

Find letters existing in both forms

Count Result

Return count of special letters

Key Takeaway

🎯 Key Insight: Use sets to efficiently track which letters appear in both lowercase and uppercase forms

Asked in

G Google 15 M Microsoft 12

The key insight is to track which letters appear in both lowercase and uppercase forms. Best approach uses two sets in a single pass to categorize characters, then counts the intersection. Time: O(n), Space: O(1)

Common Approaches

Approach	Time	Space	Notes
✓ Brute Force - Check Each Letter Twice	O(n²)	O(1)	For each letter, search the entire string for both cases
Single Pass with Character Tracking	O(n)	O(1)	Track lowercase and uppercase occurrences in one pass

Brute Force - Check Each Letter Twice — Algorithm Steps

Find all unique letters in the string
For each letter, search string for lowercase version
For same letter, search string for uppercase version
Count letters found in both cases

Visualization

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Step-by-Step Walkthrough

Find unique letters

Identify each distinct letter

Search both cases

For each letter, search for lowercase and uppercase

Count matches

Count letters that exist in both forms

Code -

solution.c — C

#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <stdbool.h>

int solution(char* word) {
    int specialCount = 0;
    bool checked[26] = {false};
    int len = strlen(word);
    
    for (int i = 0; i < len; i++) {
        char letter = tolower(word[i]);
        int index = letter - 'a';
        
        if (!checked[index]) {
            bool hasLower = false, hasUpper = false;
            
            for (int j = 0; j < len; j++) {
                if (word[j] == letter) hasLower = true;
                if (word[j] == toupper(letter)) hasUpper = true;
            }
            
            if (hasLower && hasUpper) {
                specialCount++;
            }
            
            checked[index] = true;
        }
    }
    
    return specialCount;
}

int main() {
    char word[1001];
    fgets(word, sizeof(word), stdin);
    word[strcspn(word, "\n")] = 0;
    
    int result = solution(word);
    printf("%d\n", result);
    
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n²)

For each unique letter, we scan the entire string twice

⚠ Quadratic Growth

Space Complexity

O(1)

Only using variables, no extra data structures

✓ Linear Space

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Medium Frequency

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Smart Actions

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Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Brute Force - Check Each Letter Twice — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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