060.Permutation Sequence

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Test cases

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Solution 1: tle

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class Solution {
    public String getPermutation(int n, int k) {
        if (n < 1 || k < 1) {
            return null;
        }
        List<String> result = new ArrayList<>();
        StringBuilder sb = new StringBuilder();
        permuteHelper(result, sb, n, k);
        return result.get(k - 1).toString();
    }
    
    public void permuteHelper(List<String> result, 
                              StringBuilder sb, 
                              int n,
                              int k) {
        if (result.size() < k) {                       
          if (sb.length() == n) {
              result.add(sb.toString());
          } else {
              for (int i = 1; i <= n; i++) {
                  if (sb.indexOf(i + "") < 0) {
                      sb.append(i + "");
                      permuteHelper(result, sb, n);
                      sb.setLength(sb.length() - 1);
                  }
              }
          }
        }
    }
}