String Permutation

 1 public static List<String> permutate(String str) {
 2     List<String> res = new ArrayList<String>();
 3     if(str == null || str.length() == 0) return res;
 4     
 5     StringBuffer sb = new StringBuffer(str);
 6     int[] idx = new int[str.length()];
 7     
 8     int low = ‘a‘, high = ‘z‘;
 9     int start = 0;
10     for(int i=0; i<str.length(); i++) {
11         if(str.charAt(i) <= high && str.charAt(i) >= low) {
12             idx[start] = i;
13             start++;
14         }
15     }
16     
17     System.out.println(start);
18 
19     boolean[] visited = new boolean[start];
20     helper(str, res, sb, idx, visited, 0, start);
21     return res;
22 }
23 
24 public static void helper(String str, List<String> res, StringBuffer sb, int[] idx, boolean[] visited, int pos, int end) {
25     if(pos == end) {
26         res.add(new StringBuffer(sb).toString());
27         return;
28     }
29             
30     for(int i=0; i<end; i++) {
31         if(visited[i] == false) {
32             visited[i] = true;
33             sb.setCharAt(idx[pos], str.charAt(idx[i]));
34             helper(str, res, sb, idx, visited, pos+1, end);
35             visited[i] = false;
36         }
37     }
38 }