784. Letter Case Permutation

784. Letter Case Permutation

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.

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

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

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

Note:

• S will be a string with length at most 12.

• S will consist only of letters or digits.

My Solutions:

和combinations 类似，加入list的条件是index == s.length(), 即所有char已经加入

如果char是letter，需要lower case和upper case都dfs一遍

方法1：和模板类似

class Solution {
    public List<String> letterCasePermutation(String S) {
        List<String> list = new ArrayList<>();
        dfs(S, new StringBuilder(), list, 0);
        return list;
    }
    
    public void dfs(String s, StringBuilder sb, List<String> list, int pos) {
        if (pos == s.length()) {
            list.add(sb.toString());
            return;
        }
        
        char c = s.charAt(pos);
        if (Character.isDigit(c)) {
            sb.append(c);
            dfs(s, sb, list, pos + 1);
        } else {
            sb.append(Character.toLowerCase(c));
            dfs(s, sb, list, pos + 1);
            sb.deleteCharAt(sb.length() - 1);
            sb.append(Character.toUpperCase(c));
            dfs(s, sb, list, pos + 1);
        }
        sb.deleteCharAt(sb.length() - 1);
    }
}

方法2：把s变成char[] arr

class Solution {
    public List<String> letterCasePermutation(String S) {
        List<String> list = new ArrayList<>();
        dfs(list, S.toCharArray(), 0);
        return list;
    }
    
    private void dfs(List<String> list, char[] arr, int index) {
        if (index == arr.length) {
            list.add(new String(arr));
            return;
        }
        if (Character.isLetter(arr[index])) {
            arr[index] = Character.toLowerCase(arr[index]);
            dfs(list, arr, index + 1);
            arr[index] = Character.toUpperCase(arr[index]);
        }
        dfs(list, arr, index + 1);
    }
}