LeetCode 784. Letter Case Permutation

原题链接在这里：https://leetcode.com/problems/letter-case-permutation/

题目：

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 between 1 and 12.
• S will consist only of letters or digits.

题解：

For each dfs level, state needs current index and current item.

When the index hits the end, add to res.

Time Complexity: O(exponential).

Space: O(S.lengt()).

AC Java:

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

类似Combinations.