Explanation: For the sample test case, we have 2 strings "aab" and "aac". S1 = {"a", "aa", "aab", "ab", "b"} . These are the 5 unique substrings of "aab". S2 = {"a", "aa", "aac", "ac", "c" } . These are the 5 unique substrings of "aac". Now, S = {S1 U S

Solution.java

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.List;
import java.util.Set;
import java.util.TreeSet;

public class Solution {
	
	public Set<String> getSubStrings(String s,Set<String> result) {
		for(int i=1;i<=s.length();i++) {
			for(int j=0;j<s.length();j++) {
				int count = i;
				int tmpIndex = j;
				String res = ""; 
				while(count>0) {
					if(tmpIndex>=s.length())
						break;
					res += s.charAt(tmpIndex++);
					count--;
				}
				result.add(res);
			}
		}
		return result;
	}
	
	public static void main(String[] args) throws IOException {
		Solution solution = new Solution();
		int n = Integer.valueOf(new BufferedReader(new InputStreamReader(System.in)).readLine());
		String[] data = new String[n];
		Set<String> sub = new TreeSet<String>();
		for(int i=0;i<n;i++) {
			data[i] = new BufferedReader(new InputStreamReader(System.in)).readLine();
		}
		int q = Integer.valueOf(new BufferedReader(new InputStreamReader(System.in)).readLine());
		int[] k = new int[q];
		for(int i=0;i<q;i++) {
			k[i] = Integer.valueOf(new BufferedReader(new InputStreamReader(System.in)).readLine());
		}
		
		
		for(int i=0;i<n;i++) {
			sub = solution.getSubStrings(data[i], sub);
		}
		List<String> inList = new ArrayList<String>(sub);
		int size = inList.size();
		for(int i : k) {
			if(i<=size&&i>0) {
				System.out.println(inList.get(i-1));
			}else {
				System.out.println("INVALID");
			}
		}
		
	}
}

Explanation:

For the sample test case, we have 2 strings "aab" and "aac".
S1 = {"a", "aa", "aab", "ab", "b"} . These are the 5 unique substrings of "aab".
S2 = {"a", "aa", "aac", "ac", "c" } . These are the 5 unique substrings of "aac".
Now, S = {S1 U S2} = {"a", "aa", "aab", "aac", "ab", "ac", "b", "c"}. Totally, 8 unique strings are present in the set 'S'.
The lexicographically 3rd smallest string in 'S' is "aab" and the lexicographically 8th smallest string in 'S' is "c". Since there are only 8 distinct substrings, the answer to the last query is "INVALID".

I just can not pass the test case
https://www.interviewstreet.com/challenges/dashboard/#problem/4efa210eb70ac