784. 字母大小写全排列

 1 public class Solution {
 2     private int len = -1;
 3     private char[] s = null;
 4     private void helper(int cur, List<String> res){
 5         if (cur >= len) {
 6             res.add(String.valueOf(s));
 7             return;
 8         }
 9         if (!Character.isDigit(s[cur])){
10             helper(cur+1,res);
11             if (s[cur] >= ‘a‘ && s[cur] <= ‘z‘){
12                 s[cur] -= 32;
13                 helper(cur+1,res);
14             } else {
15                 s[cur] += 32;
16                 helper(cur+1,res);
17             }
18         } else
19             helper(cur+1, res);
20     }
21 
22     public List<String> letterCasePermutation(String S) {
23         len = S.length();
24         s = S.toCharArray();
25         List<String> res = new ArrayList<>();
26         helper(0,res);
27         return res;
28     }
29 
30     public static void main(String[] args) {
31         List<String> list = new Solution().letterCasePermutation("12345");
32         System.out.println("list = " + list);
33     }
34 }