import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.util.stream.IntStream;
import java.util.Arrays;
import java.util.PriorityQueue;
import java.io.BufferedWriter;
import java.util.InputMismatchException;
import java.io.IOException;
import java.util.stream.Stream;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author lewin
 */
public class string_cutting_lewin_wa {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        OutputWriter out = new OutputWriter(outputStream);
        StringCutting solver = new StringCutting();
        int testCount = in.nextInt();
        for (int i = 1; i <= testCount; i++)
            solver.solve(i, in, out);
        out.close();
    }

    static class StringCutting {
        public void solve(int testNumber, InputReader in, OutputWriter out) {
            int k = in.nextInt();
            String s = in.next();
            StringBuilder ans = new StringBuilder();
            for (char let = 'z'; k > 0 && let >= 'a'; let--) {
                int rr = 0;
                while (rr < s.length() && s.charAt(rr) == let) {
                    rr++;
                    ans.append(let);
                }
                s = s.substring(rr);

                int needcuts = 0;
                int count = 0;
                int last = -1;
                for (int i = 1; i < s.length(); i++) {
                    if (s.charAt(i) == let) {
                        count++;
                        last = i;
                    }
                    if ((s.charAt(i) == let || s.charAt(i - 1) == let) && (s.charAt(i) != s.charAt(i - 1))) needcuts++;
                }
                needcuts = (needcuts + 1) / 2;

                if (needcuts <= k) {
                    k -= needcuts;
                    for (int i = 0; i < count; i++) ans.append(let);
                    s = s.substring(last + 1);
                } else {
                    int[] maxd = new int[s.length() + 1];
                    int[] suff = SuffixArray2.suffixArray(s);
                    int[] mxs = new int[s.length() + 1];
                    mxs[s.length()] = -1;
                    int[] whichs = new int[s.length() + 1];
                    int[] rank = AUtils.inverse(suff);
                    int c = 0;
                    for (int i = s.length() - 1; i >= 0; i--) {
                        if (s.charAt(i) == let) {
                            c++;
                        } else {
                            c = 0;
                        }
                        maxd[i] = Math.max(c, maxd[i + 1]);
                        if (rank[i] > mxs[i + 1]) {
                            mxs[i] = rank[i];
                            whichs[i] = i;
                        } else {
                            mxs[i] = mxs[i + 1];
                            whichs[i] = whichs[i + 1];
                        }
                    }

                    PriorityQueue<Integer> pq = new PriorityQueue<>();
                    int sum = 0;
                    c = 0;
                    int mx = 0, tt = 0;
                    int suffix = 0;
                    for (int i = 0; i < s.length(); i++) {
                        if (s.charAt(i) == let) {
                            c++;
                        } else {
                            pq.add(c);
                            sum += c;
                            if (pq.size() > k - 1) sum -= pq.poll();
                            c = 0;
                        }
                        if (sum + maxd[i + 1] > mx) {
                            mx = sum + maxd[i + 1];
                            tt = sum;
                            suffix = whichs[i + 1];
                        }
                    }
                    for (int i = 0; i < tt; i++) {
                        ans.append(let);
                    }
                    s = s.substring(suffix);
                    break;
                }
            }
            out.println(ans + s);
        }

    }

    static class OutputWriter {
        private final PrintWriter writer;

        public OutputWriter(OutputStream outputStream) {
            writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
        }

        public OutputWriter(Writer writer) {
            this.writer = new PrintWriter(writer);
        }

        public void print(Object... objects) {
            for (int i = 0; i < objects.length; i++) {
                if (i != 0) {
                    writer.print(' ');
                }
                writer.print(objects[i]);
            }
        }

        public void println(Object... objects) {
            print(objects);
            writer.println();
        }

        public void close() {
            writer.close();
        }

    }

    static class SuffixArray2 {
        public static int[] suffixArray(CharSequence s) {
            int n = s.length();
            Integer[] sa = IntStream.range(0, n).mapToObj(Integer::valueOf).toArray(Integer[]::new);
            int[] rank = s.chars().toArray();

            for (int len = 1; len < n; len *= 2) {
                long[] rank2 = new long[n];
                for (int i = 0; i < n; i++)
                    rank2[i] = ((long) rank[i] << 32) + (i + len < n ? rank[i + len] + 1 : 0);

                Arrays.sort(sa, (a, b) -> Long.compare(rank2[a], rank2[b]));

                for (int i = 0; i < n; i++)
                    rank[sa[i]] = i > 0 && rank2[sa[i - 1]] == rank2[sa[i]] ? rank[sa[i - 1]] : i;
            }
            return Arrays.stream(sa).mapToInt(Integer::intValue).toArray();
        }

    }

    static class AUtils {
        public static int[] inverse(int[] arr) {
            int n = arr.length;
            int[] ret = new int[n];
            for (int i = 0; i < n; i++) {
                ret[arr[i]] = i;
            }
            return ret;
        }

    }

    static class InputReader {
        private InputStream stream;
        private byte[] buf = new byte[1 << 16];
        private int curChar;
        private int numChars;

        public InputReader(InputStream stream) {
            this.stream = stream;
        }

        public int read() {
            if (this.numChars == -1) {
                throw new InputMismatchException();
            } else {
                if (this.curChar >= this.numChars) {
                    this.curChar = 0;

                    try {
                        this.numChars = this.stream.read(this.buf);
                    } catch (IOException var2) {
                        throw new InputMismatchException();
                    }

                    if (this.numChars <= 0) {
                        return -1;
                    }
                }

                return this.buf[this.curChar++];
            }
        }

        public int nextInt() {
            int c;
            for (c = this.read(); isSpaceChar(c); c = this.read()) {
                ;
            }

            byte sgn = 1;
            if (c == 45) {
                sgn = -1;
                c = this.read();
            }

            int res = 0;

            while (c >= 48 && c <= 57) {
                res *= 10;
                res += c - 48;
                c = this.read();
                if (isSpaceChar(c)) {
                    return res * sgn;
                }
            }

            throw new InputMismatchException();
        }

        public String next() {
            int c;
            while (isSpaceChar(c = this.read())) {
                ;
            }

            StringBuilder result = new StringBuilder();
            result.appendCodePoint(c);

            while (!isSpaceChar(c = this.read())) {
                result.appendCodePoint(c);
            }

            return result.toString();
        }

        public static boolean isSpaceChar(int c) {
            return c == 32 || c == 10 || c == 13 || c == 9 || c == -1;
        }

    }
}