Language: Java

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.Collection;
import java.util.HashMap;
import java.util.Map;
import java.util.StringTokenizer;
import java.util.stream.Stream;

public class Solution implements Runnable {
	
	static final double time = 1e9;
	static final int MOD = (int) 1e9 + 7;
	static final long mh = Long.MAX_VALUE;
	static final Reader in = new Reader();
	static final PrintWriter out = new PrintWriter(System.out);
	StringBuilder answer = new StringBuilder();

	public static void main(String[] args) {
		new Thread(null, new Solution(), "persefone", 1 << 28).start();
	}

	@Override
	public void run() {
		long start = System.nanoTime();
		solve();		
		printf();
		long elapsed = System.nanoTime() - start;
//		out.println("stamp : " + elapsed / time);
//		out.println("stamp : " + TimeUnit.SECONDS.convert(elapsed, TimeUnit.NANOSECONDS));
		close();
	}

	
	void solve() {
		int n = in.nextInt();
		char[] s = in.next().toCharArray();
		Map<Node, Integer> map = new HashMap<>();
		for (int i = 0; i < 1 << n; i++) {
			String c1 = new String("");
			String c2 = new String("");
			for (int j = 0; j < n; j++) {
				if ((i & (1 << j)) > 0) {
					c1 += s[j];
				} else {
					c2 += s[j];
				}
			}
			Node node = new Node(c1, c2);
			map.put(node, map.getOrDefault(node, 0) + 1);
		}
	
		long ans = 0;
		for (int i = 0; i < 1 << n; i++) {
			String c1 = new String("");
			String c2 = new String("");
			for (int j = 1; j <= n; j++) {
				if ((i & (1 << j)) > 0) {
					c1 += s[2 * n - j];
				} else {
					c2 += s[2 * n - j];
				}
			}
			Node node = new Node(c1, c2);
			ans += map.getOrDefault(node, 0);
			
		}
		printf(ans);
  	}
	
	static class Node {
		String sb, sd;
		Node(String sb, String sd) {
			this.sb = sb; 
			this.sd = sd;
		}
		@Override 
		public int hashCode() {
			int hash = 31;
			hash += 59 * sb.hashCode();
			hash += 59 * sd.hashCode();
			return hash;
		}
		
		@Override
		public boolean equals(Object o) {
			if (o == null || !(o instanceof Node)) return false;
			Node node = (Node) o;
			return sb.equals(node.sb) && sd.equals(node.sd);
		}
		
		@Override 
		public String toString() {
			return sb.concat(" ").concat(sd).toString();
		}
		
		
		
	}
			
	
	public interface Hash {
		public void computeHashArray();
		public void computeModArray();
		
	}
	
	class StringHash implements Hash {
		// length of string s
		private final long MH = Long.MAX_VALUE;
		private int n;
		private char[] ch;
		private long[] hash, mod;
		StringHash(char[] ch) {
			this.ch = ch;
			n = ch.length;
			hash = new long[n + 1];
			mod = new long[n + 1];
			computeHashArray();
			computeModArray();
		}
		
		StringHash(String s) {
			this(s.toCharArray());
		}
		
		StringHash(CharSequence s) {
			this(s.toString());
		}
		
		@Override
		public void computeModArray() {
			mod[0] = 1;
			for (int i = 1; i <= n; i++) mod[i] = (mod[i - 1] * 53) % MH;
		}
		
		@Override
		public void computeHashArray() {
			for (int i = 0; i < n; i++) hash[i + 1] = (hash[i] * 53 + ch[i] - 'a') % MH;
		}
		
		public long getHash(int i, int j) {
			return (hash[j] - hash[i] * mod[j - i] + MH * MH) % MH;
		}
		
		public long getHash(String s) {
			long h = 0;
			for (int i = 0; i < s.length(); i++) h = (h * 53 + s.charAt(i) - 'a') % MH;
			return h;
		}
		
		public long[] getHashArray() { return hash; }
		public long[] getModArray() { return mod; }
	}
	
	public interface Manacher {
		public void oddPalindrome();
		public void evenPalindrome();
	}
	
	class Palindrome implements Manacher {
		private int n;
		private char[] ch;
		private int[] oddPal, evenPal;
		public Palindrome() {}
		public Palindrome(String s) {
			this(s.toCharArray());
		}
		public Palindrome(char[] ch) {
			this.ch = ch;
			n = ch.length;
			oddPal = new int[ch.length];
			evenPal = new int[ch.length];
			oddPalindrome();
			evenPalindrome();
		}
		
		@Override
		public void oddPalindrome() {
			for (int i = 0, l = 0, r = -1; i < n; i++) {
				int k = i > r ? 1 : Math.min(oddPal[r + l - i], r - i);
				while (0 <= i - k && i + k < n && ch[i - k] == ch[i + k]) k++;
				oddPal[i] = k--;
				if (i + k > r) { 
					l = i - k;
					r = i + k;
				}
			}
		}
		@Override
		public void evenPalindrome() {
			for (int i = 0, l = 0, r = -1; i < n; i++) {
				int k = i > r ? 0 : Math.min(evenPal[r + l - i + 1], r - i + 1);
				while (0 <= i - k - 1 && i + k < n && ch[i - k - 1] == ch[i + k]) k++;
				evenPal[i] = k--;
				if (i + k > r) {
					l = i - k - 1;
					r = i + k;
				}
			}
		}
	}
	
	
	void printf() {
		out.print(answer);
	}
	
	void close() {
		out.close();
	}

	void printf(Stream<?> str) {
		str.forEach(o -> add(o, " "));
		add("\n");
	}

	 
	void printf(Object... obj) {
		printf(false, obj);
	}

	void printfWithDescription(Object... obj) {
		printf(true, obj);
	}
	
		
	private void printf(boolean b, Object... obj) {

		if (obj.length > 1) {
			for (int i = 0; i < obj.length; i++) {
				if (b) add(obj[i].getClass().getSimpleName(), " - ");
				if (obj[i] instanceof Collection<?>) {
					printf((Collection<?>) obj[i]);
				} else if (obj[i] instanceof int[][]) {
					printf((int[][])obj[i]);
				} else if (obj[i] instanceof long[][]) {
					printf((long[][])obj[i]);
				} else if (obj[i] instanceof double[][]) {
					printf((double[][])obj[i]);
				} else printf(obj[i]);
			}
			return;
		}

		if (b) add(obj[0].getClass().getSimpleName(), " - ");
		printf(obj[0]);
	}

	void printf(Object o) {
		if (o instanceof int[])
			printf(Arrays.stream((int[]) o).boxed());
		else if (o instanceof char[])
			printf(new String((char[]) o));
		else if (o instanceof long[])
			printf(Arrays.stream((long[]) o).boxed());
		else if (o instanceof double[])
			printf(Arrays.stream((double[]) o).boxed());
		else if (o instanceof boolean[]) {
			for (boolean b : (boolean[]) o) add(b, " ");
			add("\n");
		}
		else 
			add(o, "\n");
	}

	void printf(int[]... obj) {
		for (int i = 0; i < obj.length; i++) printf(obj[i]);
	}
	
	void printf(long[]... obj) {
		for (int i = 0; i < obj.length; i++) printf(obj[i]);
	}
	
	void printf(double[]... obj) {
		for (int i = 0; i < obj.length; i++) printf(obj[i]);
	}
	
	void printf(boolean[]... obj) {
		for (int i = 0; i < obj.length; i++) printf(obj[i]);
	}
	
	void printf(Collection<?> col) {
		printf(col.stream());
	}

	<T, K> void add(T t, K k) {
		if (t instanceof Collection<?>) {
			((Collection<?>) t).forEach(i -> add(i, " "));
		} else if (t instanceof Object[]) {
			Arrays.stream((Object[]) t).forEach(i -> add(i, " "));
		} else
			add(t);
		add(k);
	}
	
	
	
	<T> void add(T t) {
		answer.append(t);
	}

	@SuppressWarnings("unchecked")
	<T extends Comparable<? super T>> T min(T... t) {
		if (t.length == 0)
			return null;
		T m = t[0];
		for (int i = 1; i < t.length; i++)
			if (t[i].compareTo(m) < 0)
				m = t[i];
		return m;
	}

	@SuppressWarnings("unchecked")
	<T extends Comparable<? super T>> T max(T... t) {
		if (t.length == 0)
			return null;
		T m = t[0];
		for (int i = 1; i < t.length; i++)
			if (t[i].compareTo(m) > 0)
				m = t[i];
		return m;
	}

	int gcd(int a, int b) {
		return (b == 0) ? a : gcd(b, a % b);
	}

	long gcd(long a, long b) {
		return (b == 0) ? a : gcd(b, a % b);
	}
	
	// c = gcd(a, b) -> extends gcd method: ax + by = c <----> (b % a)p + q = c;
	int[] ext_gcd(int a, int b) {
		if (b == 0) return new int[] {a, 1, 0 };
		int[] vals = ext_gcd(b, a % b);
		int d = vals[0]; // gcd
	    int p = vals[2]; // 
		int q = vals[1] - (a / b) * vals[2];
		return new int[] { d, p, q };	  
	}
	
	// find any solution of the equation: ax + by = c using extends gcd
	boolean find_any_solution(int a, int b, int c, int[] root) {
		int[] vals = ext_gcd(Math.abs(a), Math.abs(b));
		if (c % vals[0] != 0) return false;
		printf(vals);
		root[0] = c * vals[1] / vals[0];
		root[1] = c * vals[2] / vals[0];
		if (a < 0) root[0] *= -1;
		if (b < 0) root[1] *= -1;
		return true;
	}
	
	int mod(int x) { return x % MOD; }
	
	int mod(int x, int y) { return mod(mod(x) + mod(y)); }
	
	long mod(long x) { return x % MOD; }
	
	long mod (long x, long y) { return mod(mod(x) + mod(y)); }
	
	int lw(long[] f, int l, int r, long k) {
		int R = r, m = 0;
		while (l <= r) {
			m = l + r >> 1;
		    if (m == R) return m;
		    if (f[m] >= k) r = m - 1; else l = m + 1;
		}
		return l;
	}
	
	int up(long[] f, int l, int r, long k) {
		int R = r, m = 0;
		while (l <= r) {
			m = l + r >> 1;
		    if (m == R) return m;
		    if (f[m] > k) r = m - 1; else l = m + 1;
		}
		return l;
	}
	
	int lw(int[] f, int l, int r, int k) {
//		int R = r, m = 0;
		while (l <= r) {
			int m = l + r >> 1;
//		    if (m == R) return m;
		    if (f[m] >= k) r = m - 1; else l = m + 1;
		}
		return l;
	}
	
	int up(int[] f, int l, int r, int k) {
		int R = r, m = 0;
		while (l <= r) {
			m = l + r >> 1;
		    if (m == R) return m;
		    if (f[m] > k) r = m - 1; else l = m + 1;
		}
		return l;
	}

	long calc(int base, int exponent) {
		if (exponent == 0) return 1;
		long m = calc(base, exponent >> 1);
		if (exponent % 2 == 0) return m * m % MOD;
		return base * m * m % MOD;
	}
	
	long power(int base, int exponent) {
		if (exponent == 0) return 1;
		long m = power(base, exponent >> 1);
		if (exponent % 2 == 0) return m * m;
		return base * m * m;
	}
	
	void swap(int[] a, int i, int j) {
		a[i] ^= a[j];
		a[j] ^= a[i];
		a[i] ^= a[j];
	}
	
	void swap(long[] a, int i, int j) {
		long tmp = a[i];
		a[i] = a[j];
		a[j] = tmp;
	}
	
	static class Pair<K extends Comparable<? super K>, V extends Comparable<? super V>>
			implements Comparable<Pair<K, V>> {
		private K k;
		private V v;

		Pair() {}

		Pair(K k, V v) {
			this.k = k;
			this.v = v;
		}

		K getK() { return k; }

		V getV() { return v; }

		void setK(K k) { this.k = k; }

		void setV(V v) { this.v = v; }

		void setKV(K k, V v) {
			this.k = k;
			this.v = v;
		}

		@SuppressWarnings("unchecked")
		@Override
		public boolean equals(Object o) {
			if (this == o) return true;
			if (o == null || !(o instanceof Pair)) return false;
			Pair<K, V> p = (Pair<K, V>) o;
			return k.compareTo(p.k) == 0 && v.compareTo(p.v) == 0;
		}

		@Override
		public int hashCode() {
			int hash = 31;
			hash = hash * 89 + k.hashCode();
			hash = hash * 89 + v.hashCode();
			return hash;
		}

		@Override
		public int compareTo(Pair<K, V> pair) {
			return k.compareTo(pair.k) == 0 ? v.compareTo(pair.v) : k.compareTo(pair.k);
		}

		@Override
		public Pair<K, V> clone() {
			return new Pair<K, V>(this.k, this.v);
		}

		@Override
		public String toString() {
			return String.valueOf(k).concat(" ").concat(String.valueOf(v)).concat("\n");
		}
	}

	static class Reader {
		private BufferedReader br;
		private StringTokenizer st;

		Reader() {
			br = new BufferedReader(new InputStreamReader(System.in));
		}

		String next() {
			try {
				while (st == null || !st.hasMoreTokens()) {
					st = new StringTokenizer(br.readLine());
				}
			} catch (IOException e) {
				e.printStackTrace();
			}
			return st.nextToken();
		}

		int nextInt() {
			return Integer.parseInt(next());
		}

		int[] nextIntArray(int n) {
			int[] arr = new int[n];
			for (int i = 0; i < n; i++)
				arr[i] = nextInt();
			return arr;
		}

		long nextLong() {
			return Long.parseLong(next());
		}

		String nextLine() {
			String s = "";
			try {
				s = br.readLine();
			} catch (IOException e) {
				e.printStackTrace();
			}
			return s;
		}
	}
}

Compiler output:
Main.java:12: error: class Solution is public, should be declared in a file named Solution.java
public class Solution implements Runnable {
       ^
1 error