Greatest English Letter in Upper and Lower Case - Problem

Given a string of English letters s, return the greatest English letter which occurs as both a lowercase and uppercase letter in s. The returned letter should be in uppercase. If no such letter exists, return an empty string.

An English letter b is greater than another letter a if b appears after a in the English alphabet.

Input & Output

Example 1 — Basic Case

$ Input: s = "lEeTcOdE"

› Output: "E"

💡 Note: The string contains both 'E' and 'e'. Among all letters that appear in both cases (E, T, C, O, D), E is the greatest alphabetically.

Example 2 — No Valid Letters

$ Input: s = "arRAzFif"

› Output: "R"

💡 Note: The string contains both 'R' and 'r'. R is the only letter that appears in both uppercase and lowercase forms.

Example 3 — Single Letter Case

$ Input: s = "AbCdEfGhIjKlMnOpQrStUvWxYz"

› Output: ""

💡 Note: Each letter appears only once in either uppercase or lowercase, but never both. No letter appears in both cases.

Constraints

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

Visualization

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Asked in

G Google 15 a Amazon 12 M Microsoft 8

The key insight is to find letters that appear in both uppercase and lowercase, then return the greatest one alphabetically. Best approach uses a hash set to collect characters in O(n) time, then checks pairs from Z to A. Time: O(n), Space: O(1).

Common Approaches

✓ Brute Force

⏱️ Time: O(n) Space: O(1)

Iterate through the alphabet from Z to A, and for each letter check if both uppercase and lowercase versions exist in the string. Return the first match found.

Single Pass with Bitmask

⏱️ Time: O(n) Space: O(1)

Use bit manipulation to track which uppercase and lowercase letters we've seen. Use two integers as bitmasks - one for uppercase, one for lowercase. Then check pairs from Z to A using bitwise AND.

Two Pass with Hash Set

⏱️ Time: O(n) Space: O(1)

Use a hash set to collect all characters in the string during the first pass. Then iterate from Z to A checking if both cases of each letter exist in the set.

Brute Force — Algorithm Steps

Step 1: Start from letter Z and go backwards to A
Step 2: For each letter, check if both upper and lower case exist in string
Step 3: Return first letter found, or empty string if none

Visualization

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Step-by-Step Walkthrough

Start from Z

Check if both 'Z' and 'z' exist

Continue backwards

Check Y, X, W... until found

Return first match

First letter with both cases

Code -

solution.c — C

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

char* solution(char* s) {
    static char result[2];
    
    // Check from Z to A (reverse alphabetical order)
    for (char c = 'Z'; c >= 'A'; c--) {
        char upperChar = c;
        char lowerChar = c + 32;
        
        // Check if both upper and lower case exist in string
        int hasUpper = (strchr(s, upperChar) != NULL);
        int hasLower = (strchr(s, lowerChar) != NULL);
        
        if (hasUpper && hasLower) {
            result[0] = upperChar;
            result[1] = '\0';
            return result;
        }
    }
    
    result[0] = '\0';
    return result;
}

int main() {
    char s[1001];
    fgets(s, sizeof(s), stdin);
    s[strcspn(s, "\n")] = 0;  // Remove newline
    
    char* result = solution(s);
    printf("%s\n", result);
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n)

We check at most 26 letters, and for each letter we scan the string once, giving O(26n) = O(n)

✓ Linear Growth

Space Complexity

O(1)

Only using constant extra variables, no additional data structures

✓ Linear Space

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Greatest English Letter in Upper and Lower Case - Problem

Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Brute Force — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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