【剑指offer】-字符串的排列-26/67

public ArrayList<String> Permutation(String str) {
    ArrayList<String> strings = new ArrayList<String>();
    if (str == null || str.length() == 0) {
      return strings;
    }
    char[] string = str.toCharArray();
    perm(strings, string, 0, string.length - 1);
    Collections.sort(strings);
    return strings;
  }
  public static void perm(ArrayList<String> strings, char[] a, int k, int m ) {
    if (k == m) {
      String str = String.valueOf(a);
      if (strings.contains(str) == false) {
        strings.add(str);
      }
    } else {
      /*
       * 进入时 
       */
      for (int i = k; i <= m; i++) {
        swap(a, i, k);
        perm(strings, a, k + 1, m);
        // 回溯
        swap(a, i, k);
      }
    }
  }
  public static void swap(char[] a, int x1, int x2) {
    char temp;
    temp = a[x1];
    a[x1] = a[x2];
    a[x2] = temp;
  }