# K-th lexicographically smallest unique substring of a given string

Given a string S. The task is to print the K-th lexicographically smallest one among the different substrings of s.

A substring of s is a string obtained by taking out a non-empty contiguous part in s.
For example, if s = ababc, a, bab and ababc are substrings of s, while ac, z and an empty string are not. Also, we say that substrings are different when they are different as strings.

Examples:

Input: str = “aba”, k = 4
Output: b
All unique substrings are a, ab, aba, b, ba.
Thus the 4th lexicographically smallest substring is b.

Input: str = “geeksforgeeks”, k = 5
Output: eeksf

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Approach: For an arbitrary string t, each of its proper suffix is lexicographically smaller than t, and the lexicographic
rank of t is at least |t|. Thus, the length of the answer is at most K.
Generate all substrings of s whose lengths are at most K. Sort them, unique them, and print the K-th one, where N = |S|.

Below is the implementation of the above approach:

 # Python implementation of the above approach def kThLexString(st, k, n):     # Set to store the unique substring     z = set()         for i in range(n):         # String to create each substring         pp = ""             for j in range(i, i + k):                if (j >= n):                    break                pp += s[j]             # adding to set             z.add(pp)         # converting set into a list     fin = list(z)             # sorting the strings int the list     # into lexicographical order     fin.sort()         # printing kth substring     print(fin[k - 1])        s = "geeksforgeeks"    k = 5    n = len(s) kThLexString(s, k, n)

Output:

eeksf

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