Substrings.java

import java.io.*;
import java.util.*;

public class Substrings {
    static boolean DEBUG = false;

    static Set<String> substrings(String source){
        Set<String> substrings = new TreeSet<>();
        for (int i = 0; i < source.length(); i++) {
            for (int j = i + 1; j <= source.length(); j++) {
                substrings.add(source.substring(i, j));
            }
        }
        return substrings;
    }

    static String concat(Collection<String> strings){
        StringBuilder sb = new StringBuilder();
        for (String string: strings){
            sb.append(string);
        }
        return sb.toString();
    }

    static char test(String source, long elt){
        Set<String> substrings = substrings(source);
        if (DEBUG) System.out.println(substrings);
        long count = 0;
        for(String string: substrings){
            count += string.length();
            if (count < elt){
                continue;
            } else if (count == elt){
                return string.charAt(string.length() - 1);
            } else if (count > elt) {
                return string.charAt((int)(count - elt));
            }
        }
        throw new RuntimeException("Requested element is too big " + elt + ", " + count);
    }

    public static void main(String[] args) throws FileNotFoundException {
        if (args.length > 0){
            String fileName = args[0];
            System.out.println("Reading input from file " + fileName);
            System.setIn(new FileInputStream(new File(fileName)));
        }
        System.out.println(test("dbac",3));

        Scanner s = new Scanner(System.in);
        int numTests = s.nextInt();
        String source;
        int k;
        while(numTests-- > 0){
            source = s.next();
            k = s.nextInt() - 1; //Compensate for 1-based input
            System.out.println(test(source, k));
        }
    }
}