Problem:
Score : 600 points

Problem Statement

There are 2N people numbered 1 through 2N.
The height of Person i is h_i.

How many ways are there to make N pairs of people such that the following conditions are satisfied?
Compute the answer modulo 998,244,353.

Each person is contained in exactly one pair.

For each pair, the heights of the two people in the pair are different.

Two ways are considered different if for some p and q, Person p and Person q are paired in one way and not in the other.

Constraints

1 \leq N \leq 50,000

1 \leq h_i \leq 100,000

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N
h_1
:
h_{2N}

Output

Print the answer.

Sample Input 1

2
1
1
2
3

Sample Output 1

2

There are two ways:

Form the pair (Person 1, Person 3) and the pair (Person 2, Person 4).

Form the pair (Person 1, Person 4) and the pair (Person 2, Person 3).

Sample Input 2

5
30
10
20
40
20
10
10
30
50
60

Sample Output 2

516

Solution in C# (.NET Core 3.1.201) (judge verdict: Accepted)

using System;
using System.Collections;
using System.Collections.Generic;
using System.IO;
using System.Linq;

namespace Tasks
{
    public class F
    {
        static void Main(string[] args)
        {
            var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
            Console.SetOut(sw);
            Solve();
            Console.Out.Flush();
        }

        public static void Solve()
        {
            var N = Scanner.Scan<int>();
            const int max = (int)1e5 + 1;
            const int p = 998244353;
            var F = new ModuloInteger[max];
            var inv = new ModuloInteger[max];
            var Finv = new ModuloInteger[max];
            var G = new ModuloInteger[max];
            var freq = new int[max];
            inv[1] = 1;
            for (var i = 2; i < max; i++) inv[i] = (p - p / i) * inv[p % i] % p;

            F[0] = Finv[0] = 1;
            for (var i = 1; i < max; i++)
            {
                F[i] = F[i - 1] * i;
                Finv[i] = Finv[i - 1] * inv[i];
            }

            G[0] = 1;
            for (var i = 1; i < max; i++) G[i] = G[i - 1] * (2 * i - 1);

            for (var i = 0; i < 2 * N; i++)
            {
                var H = Scanner.Scan<int>();
                freq[H]++;
            }

            var queue = new Queue<ModuloInteger[]>();
            foreach (var n in freq.Skip(1))
            {
                var list = new ModuloInteger[n / 2 + 1];
                for (var i = 0; i <= n / 2; i++)
                {
                    var tmp = F[n] * Finv[2 * i] * Finv[n - 2 * i] * G[i];
                    if (i % 2 == 1) tmp = -tmp;
                    list[i] = tmp;
                }
                queue.Enqueue(list);
            }

            while (queue.Count > 1)
            {
                var list1 = queue.Dequeue();
                var list2 = queue.Dequeue();
                queue.Enqueue(Convolution.Execute(list1, list2).ToArray());
            }

            ModuloInteger answer = 0;
            var last = queue.Dequeue();
            for (var i = 0; i < last.Length; i++) answer += last[i] * G[N - i];

            Console.WriteLine(answer);
        }

        public static class Enumeration
        {
            private static Dictionary<long, long> _memo = new Dictionary<long, long> { { 0, 1 }, { 1, 1 } };
            private static Dictionary<long, long> _modMemo = new Dictionary<long, long> { { 0, 1 }, { 1, 1 } };
            private static long _max = 1;
            private static long _modMax = 1;

            public static long Fractorial(long n)
            {
                if (_memo.ContainsKey(n)) return _memo[n];
                if (n < 0) throw new ArgumentException();
                var val = _memo[_max];
                for (var i = _max + 1; i <= n; i++)
                {
                    val *= i;
                    _memo[i] = val;
                }
                _max = n;
                return _memo[n];
            }

            public static long Fractorial(long n, long mod)
            {
                if (_modMemo.ContainsKey(n)) return _modMemo[n];
                if (n < 0) throw new ArgumentException();
                var val = _modMemo[_modMax];
                for (var i = _modMax + 1; i <= n; i++)
                {
                    val *= i % mod;
                    val %= mod;
                    _modMemo[i] = val;
                }
                _modMax = n;
                return _modMemo[n];
            }

            public static long PermutationCount(long n, long k)
            {
                if (n < k) throw new ArgumentException();
                return Fractorial(n) / Fractorial(n - k);
                // no memo
                // var ret = 1L;
                // for (var i = 0; i < k; i++) ret *= (n - i);
                // return ret;
            }

            public static long PermutationCount(long n, long k, long mod)
            {
                if (n < k) throw new ArgumentException();
                var top = Fractorial(n, mod);
                var bottom = Fractorial(n - k, mod);
                return (top * Power(bottom, mod - 2, mod)) % mod;
                // no memo
                // var ret = 1L;
                // for (var i = 0; i < k; i++) ret = (ret * (n - i) % mod) % mod;
                // return ret;
            }

            public static long CombinationCount(long n, long k)
            {
                if (n < k) throw new ArgumentException();
                k = System.Math.Min(k, n - k);
                return Fractorial(n) / (Fractorial(k) * Fractorial(n - k));
                // no memo
                // return PermutationCount(n, k) / PermutationCount(k, k);
            }

            public static long CombinationCount(long n, long k, long mod)
            {
                if (n < k) throw new ArgumentException();
                k = System.Math.Min(k, n - k);
                var top = Fractorial(n, mod);
                var bottom = (Fractorial(k, mod) * Fractorial(n - k, mod)) % mod;
                // no memo
                // var top = PermutationCount(n, k, mod);
                // var bottom = PermutationCount(k, k, mod);
                return (top * Power(bottom, mod - 2, mod)) % mod;
            }

            public static long Power(long x, long y, long mod)
            {
                var result = 1L;
                while (y > 0)
                {
                    var xmod = x % mod;
                    if ((y & 1) == 1) result = (result * xmod) % mod;
                    x = (xmod * xmod) % mod;
                    y >>= 1;
                }
                return result;
            }
        }

        public static class Math
        {
            public static long PowerMod(long x, long n, long m)
            {
                if (n < 0) throw new ArgumentException(nameof(n));
                if (m < 1) throw new ArgumentException(nameof(m));
                if (m == 1) return 0;
                uint r = 1;
                var y = (uint)(x < 0 ? x % m + m : x % m);
                var um = (uint)m;
                while (n > 1)
                {
                    if (n % 1 == 1) r = r * y % um;
                    y = y * y % um;
                    n >>= 1;
                }
                return r;
            }
        }

        public static class Convolution
        {
            private static ModuloInteger[] _sumE;
            private static ModuloInteger[] _sumIe;
            private static int _primitiveRoot;
            public static IEnumerable<ModuloInteger> Execute(IEnumerable<ModuloInteger> a, IEnumerable<ModuloInteger> b)
            {
                var (a1, b1) = (a.ToArray(), b.ToArray());
                var (n, m) = (a1.Length, b1.Length);
                var ret = new ModuloInteger[n + m - 1];
                if (System.Math.Min(n, m) <= 60)
                {
                    for (var i = 0; i < n; i++)
                        for (var j = 0; j < m; j++)
                            ret[i + j] += a1[i] * b1[j];
                    return ret;
                }
                var z = 1 << CeilPower2(n + m - 1);
                Array.Resize(ref a1, z);
                Array.Resize(ref b1, z);
                a1 = Butterfly(a1).ToArray();
                b1 = Butterfly(b1).ToArray();
                for (var i = 0; i < a1.Length; i++) a1[i] *= b1[i];
                ret = ButterflyInverse(a1).ToArray();
                Array.Resize(ref ret, n + m - 1);
                return ret.Select(x => x / z);
            }
            private static void Initialize()
            {
                if (_sumE != null && _sumIe != null) return;
                var m = ModuloInteger.Modulo;
                _primitiveRoot = PrimitiveRoot(m);
                _sumE = new ModuloInteger[30];
                _sumIe = new ModuloInteger[30];
                var es = new ModuloInteger[30];
                var ies = new ModuloInteger[30];
                var count2 = BitScanForward(m - 1);
                var e = new ModuloInteger(_primitiveRoot).Power((m - 1) >> count2);
                var ie = e.Inverse();
                for (var i = count2; i >= 2; i--)
                {
                    es[i - 2] = e;
                    ies[i - 2] = ie;
                    e *= e;
                    ie *= ie;
                }
                ModuloInteger now = 1;
                ModuloInteger inow = 1;
                for (var i = 0; i < count2 - 2; i++)
                {
                    _sumE[i] = es[i] * now;
                    _sumIe[i] = ies[i] * inow;
                    now *= ies[i];
                    inow *= es[i];
                }
            }
            private static IEnumerable<ModuloInteger> Butterfly(IEnumerable<ModuloInteger> items)
            {
                var ret = items.ToArray();
                var h = CeilPower2(ret.Length);
                Initialize();
                for (var ph = 1; ph <= h; ph++)
                {
                    var w = 1 << (ph - 1);
                    var p = 1 << (h - ph);
                    ModuloInteger now = 1;
                    for (var s = 0; s < w; s++)
                    {
                        var offset = s << (h - ph + 1);
                        for (var i = 0; i < p; i++)
                        {
                            var l = ret[i + offset];
                            var r = ret[i + offset + p] * now;
                            ret[i + offset] = l + r;
                            ret[i + offset + p] = l - r;
                        }
                        now *= _sumE[BitScanForward(~s)];
                    }
                }
                return ret;
            }
            private static IEnumerable<ModuloInteger> ButterflyInverse(IEnumerable<ModuloInteger> items)
            {
                var ret = items.ToArray();
                var h = CeilPower2(ret.Length);
                Initialize();
                for (var ph = h; ph >= 1; ph--)
                {
                    var w = 1 << (ph - 1);
                    var p = 1 << (h - ph);
                    ModuloInteger inow = 1;
                    for (var s = 0; s < w; s++)
                    {
                        var offset = s << (h - ph + 1);
                        for (var i = 0; i < p; i++)
                        {
                            var l = ret[i + offset];
                            var r = ret[i + offset + p];
                            ret[i + offset] = l + r;
                            ret[i + offset + p] = (l - r) * inow;
                        }
                        inow *= _sumIe[BitScanForward(~s)];
                    }
                }
                return ret;
            }
            private static int BitScanForward(long n)
            {
                if (n == 0) return 0;
                var x = 0;
                while ((n >> x & 1) == 0) x++;
                return x;
            }
            private static int CeilPower2(int n)
            {
                var x = 0;
                while ((1 << x) < n) x++;
                return x;
            }
            private static int PrimitiveRoot(long m)
            {
                if (_primitiveRoot != 0) return _primitiveRoot;
                switch (m)
                {
                    case 2:
                        return _primitiveRoot = 1;
                    case 167772161:
                    case 469762049:
                    case 998244353:
                        return _primitiveRoot = 3;
                    case 754974721:
                        return _primitiveRoot = 11;
                }
                var divs = new long[20];
                divs[0] = 2;
                var count = 1;
                var x = (m - 1) / 2;
                while (x % 2 == 0) x /= 2;
                for (var i = 3; (long)i * i < x; i += 2)
                {
                    if (x % i != 0) continue;
                    divs[count++] = i;
                    while (x % i == 0) x /= i;
                }
                if (x > 1) divs[count++] = x;
                for (var g = 2; ; g++)
                {
                    var ok = true;
                    for (var i = 0; i < count && ok; i++)
                        if (Math.PowerMod(g, (m - 1) / divs[i], m) == 1)
                            ok = false;
                    if (ok) return _primitiveRoot = g;
                }
            }
        }
        public readonly struct ModuloInteger
        {
            public long Value { get; }
            public const long Modulo = 998244353;
            public ModuloInteger(long data) => Value = (0 <= data ? data : data + Modulo) % Modulo;
            public static implicit operator long(ModuloInteger mint) => mint.Value;
            public static implicit operator int(ModuloInteger mint) => (int)mint.Value;
            public static implicit operator ModuloInteger(long val) => new ModuloInteger(val);
            public static implicit operator ModuloInteger(int val) => new ModuloInteger(val);
            public static ModuloInteger operator +(ModuloInteger a, ModuloInteger b) => a.Value + b.Value;
            public static ModuloInteger operator +(ModuloInteger a, long b) => a.Value + b;
            public static ModuloInteger operator +(ModuloInteger a, int b) => a.Value + b;
            public static ModuloInteger operator -(ModuloInteger a, ModuloInteger b) => a.Value - b.Value;
            public static ModuloInteger operator -(ModuloInteger a, long b) => a.Value - b;
            public static ModuloInteger operator -(ModuloInteger a, int b) => a.Value - b;
            public static ModuloInteger operator *(ModuloInteger a, ModuloInteger b) => a.Value * b.Value;
            public static ModuloInteger operator *(ModuloInteger a, long b) => a.Value * (b % Modulo);
            public static ModuloInteger operator *(ModuloInteger a, int b) => a.Value * (b % Modulo);
            public static ModuloInteger operator /(ModuloInteger a, ModuloInteger b) => a * b.Inverse();
            public static ModuloInteger operator /(ModuloInteger a, long b) => a.Value * Inverse(b);
            public static ModuloInteger operator /(ModuloInteger a, int b) => a.Value * Inverse(b);
            public static bool operator ==(ModuloInteger a, ModuloInteger b) => a.Value == b.Value;
            public static bool operator !=(ModuloInteger a, ModuloInteger b) => a.Value != b.Value;
            public bool Equals(ModuloInteger other) => Value == other.Value;
            public override bool Equals(object obj) => obj is ModuloInteger other && Equals(other);
            public override int GetHashCode() => Value.GetHashCode();
            public override string ToString() => Value.ToString();
            public ModuloInteger Inverse() => Inverse(Value);
            public static ModuloInteger Inverse(long a)
            {
                if (a == 0) return 0;
                var p = Modulo;
                var (x1, y1, x2, y2) = (1L, 0L, 0L, 1L);
                while (true)
                {
                    if (p == 1) return (x2 % Modulo + Modulo) % Modulo;
                    var div = a / p;
                    x1 -= x2 * div;
                    y1 -= y2 * div;
                    a %= p;
                    if (a == 1) return (x1 % Modulo + Modulo) % Modulo;
                    div = p / a;
                    x2 -= x1 * div;
                    y2 -= y1 * div;
                    p %= a;
                }
            }
            public ModuloInteger Power(long n) => Power(Value, n);
            public static ModuloInteger Power(ModuloInteger x, long n)
            {
                if (n < 0) throw new ArgumentException();
                var r = new ModuloInteger(1);
                while (n > 0)
                {
                    if ((n & 1) > 0) r *= x;
                    x *= x;
                    n >>= 1;
                }
                return r;
            }
        }

        public static class Scanner
        {
            private static Queue<string> queue = new Queue<string>();
            public static T Next<T>()
            {
                if (!queue.Any()) foreach (var item in Console.ReadLine().Trim().Split(" ")) queue.Enqueue(item);
                return (T)Convert.ChangeType(queue.Dequeue(), typeof(T));
            }
            public static T Scan<T>() => Next<T>();
            public static (T1, T2) Scan<T1, T2>() => (Next<T1>(), Next<T2>());
            public static (T1, T2, T3) Scan<T1, T2, T3>() => (Next<T1>(), Next<T2>(), Next<T3>());
            public static (T1, T2, T3, T4) Scan<T1, T2, T3, T4>() => (Next<T1>(), Next<T2>(), Next<T3>(), Next<T4>());
            public static (T1, T2, T3, T4, T5) Scan<T1, T2, T3, T4, T5>() => (Next<T1>(), Next<T2>(), Next<T3>(), Next<T4>(), Next<T5>());
            public static IEnumerable<T> ScanEnumerable<T>() => Console.ReadLine().Trim().Split(" ").Select(x => (T)Convert.ChangeType(x, typeof(T)));
        }
    }
}