# Problem Statement

String S is a SP-string if some non-empty prefix of S is also a suffix of S. The prefix (the suffix) can't be the whole string. For example, strings "AA" and "ABCABC" are SP-strings, but strings "AB", "ABCB" are not. String S is a N-string if it satisfies the following conditions:

• It contains only letters from the first N letters of the alphabet.
• If we append any single letter from the first N letters of the alphabet to S, the resulting string will be a SP-string.

For example, "ABBA" is a 2-string because it contains only the first 2 letters of the alphabet, and both "ABBAA" and "ABBAB" are SP-strings. There is an infinite number of N-strings, but in this problem we will consider only N-strings of minimal length (that is, N-strings of length L such that no N-string of length less than L exists).

You will be given two ints - N and K. Build all the shortest possible N-strings and sort them alphabetically. Return the substring of the K-th string from index leftIdx (inclusive) to index rightIdx (exclusive). Return "" if the K-th string contains less than rightIdx characters or the K-th string doesn't exist.

# Definition

```Class:              InterestingStrings
Method:             findSubstring
Parameters:         int, int, int, int
Returns:            string
Method signature:   string findSubstring(int N, int K, int leftIdx, int rightIdx)
(be sure your method is public)
```

# Notes

• K, leftIdx, rightIdx are 0-based.
• All letters in this problem are uppercase.

# Constraints

• N will be between 1 and 26, inclusive.
• K will be between 0 and 1000000000, inclusive.
• leftIdx will be between 0 and 1000000000, inclusive.
• rightIdx will be between (leftIdx + 1) and (leftIdx + 1000), inclusive.

# Examples

```0)

1
0
3
Returns: "BAB"

1)

1
0
0
1
Returns: "A"

2)

2
0
1
3
Returns: "BA"

3)

4
0
7
9
Returns: "DA"

4)

3
1
0
7
Returns: "ACABACA"

5)

5
1000
99999999
100000000
Returns: ""
```

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