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📚 Letter Case Permutation – Java Cheat Sheet

📌 What Is It?

The Letter Case Permutation problem involves generating all possible strings by changing the case of each letter in the input string. Digits remain unchanged.

🧱 Pattern Template

Recursive Backtracking

class Solution {
    public List<String> letterCasePermutation(String s) {
        List<String> result = new ArrayList<>();
        backtrack(0, s.toCharArray(), result);
        return result;
    }

    private void backtrack(int index, char[] chars, List<String> result) {
        if (index == chars.length) {
            result.add(new String(chars));
            return;
        }

        if (Character.isLetter(chars[index])) {
            // Change to lowercase
            chars[index] = Character.toLowerCase(chars[index]);
            backtrack(index + 1, chars, result);

            // Change to uppercase
            chars[index] = Character.toUpperCase(chars[index]);
            backtrack(index + 1, chars, result);
        } else {
            // Skip digits
            backtrack(index + 1, chars, result);
        }
    }
}

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📊 Time Complexity

• Time Complexity: O(2^N), where N is the number of letters in the input string. Each letter has two possibilities (uppercase or lowercase).
• Space Complexity: O(N), for the recursion stack.

✅ Use Cases

• Generating all possible case permutations of a string.
• Exploring all combinations of letter cases for a given input.

📘 Common LeetCode Problems

• 784. Letter Case Permutation [Easy] Backtracking

🧪 Example

Input: s = "a1b2"
Output: ["a1b2", "a1B2", "A1b2", "A1B2"]

Explanation:
- For each letter, toggle between lowercase and uppercase.
- Digits remain unchanged.

💡 Pro Tips

• Use a character array for in-place modifications to avoid creating new strings repeatedly.
• Skip digits during recursion since they don't have case permutations.
• Use backtracking to explore all valid combinations efficiently.