Count the Number of Special Characters II - Problem

You are given a string word. A letter c is called special if it appears both in lowercase and uppercase in word, and every lowercase occurrence of c appears before the first uppercase occurrence of c.

Return the number of special letters in word.

Input & Output

Example 1 — Mixed Valid Cases

$ Input: word = "aaAbcBC"

› Output: 3

💡 Note: Special characters are 'a', 'b', and 'c'. Letter 'a': lowercase at positions 0,1 before uppercase at 2. Letter 'b': lowercase at 3 before uppercase at 5. Letter 'c': lowercase at 4 before uppercase at 6.

Example 2 — Invalid Order

$ Input: word = "abc"

› Output: 0

💡 Note: No uppercase letters exist, so no letter can be special (needs both cases).

Example 3 — Constraint Violation

$ Input: word = "AbBcC"

› Output: 0

💡 Note: All letters appear in uppercase before lowercase, violating the ordering constraint.

Constraints

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

Visualization

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Asked in

G Google 45 a Amazon 32 M Microsoft 28

The key insight is tracking when lowercase letters appear after their first uppercase occurrence, making them invalid. The optimal approach uses a single pass to track state for each letter. Time: O(n), Space: O(1)

Common Approaches

✓ Brute Force - Check Each Letter Separately

⏱️ Time: O(26 × n) = O(n) Space: O(1)

Check all 26 letters individually. For each letter, find all lowercase and uppercase positions, then verify the ordering constraint.

Single Pass Optimal

⏱️ Time: O(n) Space: O(1)

Make a single pass through the string, tracking when we see the first uppercase for each letter. Mark any letter as invalid if we see lowercase after its first uppercase.

Two-Pass Hash Map

⏱️ Time: O(n) Space: O(1)

Make two passes through the string. First pass records the last lowercase and first uppercase position for each character. Second pass counts valid special characters.

Brute Force - Check Each Letter Separately — Algorithm Steps

Step 1: For each letter a-z, scan string to find all lowercase positions
Step 2: Scan string again to find all uppercase positions
Step 3: Check if lowercase positions are all before uppercase positions

Visualization

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Step-by-Step Walkthrough

Find 'b'

Scan for lowercase 'b' at position 4

Find 'B'

Scan for uppercase 'B' at position 5

Validate

Check if 4 < 5 (valid special character)

Code -

solution.c — C

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

int solution(char* word) {
    int count = 0;
    int len = strlen(word);
    
    for (char c = 'a'; c <= 'z'; c++) {
        char lowerC = c;
        char upperC = c - 'a' + 'A';
        
        bool hasLower = false, hasUpper = false;
        for (int i = 0; i < len; i++) {
            if (word[i] == lowerC) hasLower = true;
            if (word[i] == upperC) hasUpper = true;
        }
        
        if (!hasLower || !hasUpper) continue;
        
        // Find last occurrence of lowercase and first occurrence of uppercase
        int lastLower = -1;
        int firstUpper = len;
        
        for (int i = 0; i < len; i++) {
            if (word[i] == lowerC) {
                lastLower = i;
            } else if (word[i] == upperC && firstUpper == len) {
                firstUpper = i;
            }
        }
        
        if (lastLower < firstUpper) {
            count++;
        }
    }
    return count;
}

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(26 × n) = O(n)

Check 26 letters, each requiring O(n) string scan

✓ Linear Growth

Space Complexity

O(1)

Only using constant extra variables

✓ Linear Space

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Count the Number of Special Characters II - Problem

Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Brute Force - Check Each Letter Separately — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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