LeetCode 2309: Greatest English Letter in Upper and Lower Case Solution

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2309. Greatest English Letter in Upper and Lower Case

Easy

Hash Table String Enumeration

Problem Explanation

Explanation:

To solve this problem, we can iterate through the given string s and maintain a set of lowercase and uppercase letters that we have encountered. For each letter, we check if its uppercase and lowercase versions are both present in the corresponding sets. If they are, we update the result letter if the current letter is greater than the previous result letter. Finally, we return the result letter in uppercase.

Initialize an empty set for lowercase letters and uppercase letters.
Initialize a variable result as an empty string.
Iterate through each character in the input string s.
If the character is a lowercase letter, add it to the lowercase set.
If the character is an uppercase letter, add it to the uppercase set.
For each letter in the lowercase set, check if its uppercase version is also in the uppercase set.
If they are both present, update the result letter if the current letter is greater than the previous result letter.
Return the result letter in uppercase if it exists, otherwise, return an empty string.

Time Complexity:

The time complexity of this algorithm is O(n), where n is the length of the input string s.

Space Complexity:

The space complexity of this algorithm is O(1) since we are using a constant amount of extra space for storing the sets and result letter.

Solution Code

class Solution {
    public String greatestLetter(String s) {
        Set<Character> lowercaseSet = new HashSet<>();
        Set<Character> uppercaseSet = new HashSet<>();
        String result = "";
        
        for (char c : s.toCharArray()) {
            if (Character.isLowerCase(c)) {
                lowercaseSet.add(c);
            } else if (Character.isUpperCase(c)) {
                uppercaseSet.add(c);
            }
        }
        
        for (char c : lowercaseSet) {
            if (uppercaseSet.contains(Character.toUpperCase(c))) {
                if (result.isEmpty() || Character.toUpperCase(c) > result.charAt(0)) {
                    result = String.valueOf(Character.toUpperCase(c));
                }
            }
        }
        
        return result;
    }
}

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