60. Permutation Sequence

 1     public String getPermutation(int n, int k) {
 2         if(n == 0 || k == 0) {
 3             return "";
 4         }
 5         k--;
 6         List<Integer> digits = new ArrayList<Integer>();
 7         for(int i = 1; i <= n; i++) {
 8             digits.add(i);
 9         }
10         StringBuilder sb = new StringBuilder();
11         int fac = factory(n-1);
12         for(int i = n - 1; i > 0; i--) {
13             int current = k / fac;
14             k %= fac;
15             sb.append(digits.get(current));
16             digits.remove(current);
17             fac /= i;
18         }
19         sb.append(digits.get(0));
20         return sb.toString();
21     }
22     
23     private static int factory(int n) {
24         int sum = 1;
25         for(int i = 1; i <= n; i++) {
26             sum *= i;
27         }
28         return sum;
29     }