784. Letter Case Permutation

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LeetCode

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Problem

Given a string s, we can transform every letter individually to be lowercase or uppercase to create another string.

Return a list of all possible strings we could create. You can return the output in any order.

Example 1:

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

Example 2:

Input: s = "3z4"
Output: ["3z4","3Z4"]

Example 3:

Input: s = "12345"
Output: ["12345"]

Example 4:

Input: s = "0"
Output: ["0"]

Constraints:

• 1 <= s.length <= 12
• s consists of lowercase English letters, uppercase English letters, and digits.

Code

class Solution {
    public List<String> letterCasePermutation(String s) {
        List<String> res = new ArrayList<>();
        helper(res, s, 0, "");
        return res;
    }

    private void helper(List<String> res, String s, int start, String curr) {
        if (start > s.length()) return;
        if (curr.length() == s.length()) {
            res.add(curr);
            return;
        }

        for (int i = start; i < s.length(); i++) {
            char c = s.charAt(i);
            if (Character.isDigit(c)) {
                curr += c;
                if (curr.length() == s.length()) {
                    res.add(curr);
                    return;
                }
                continue;
            } else {
                helper(res, s, i + 1, curr + Character.toLowerCase(c));
                helper(res, s, i + 1, curr + Character.toUpperCase(c));
            }
        }
    }
}