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 Kotlin (1.3.71) (judge verdict: Accepted)

@file:Suppress("NOTHING_TO_INLINE", "EXPERIMENTAL_FEATURE_WARNING", "OVERRIDE_BY_INLINE")
//@file:OptIn(ExperimentalStdlibApi::class)

import java.io.PrintWriter
import java.util.PriorityQueue
import java.util.TreeMap
import kotlin.math.*
import kotlin.random.Random
import kotlin.collections.sort as _sort
import kotlin.collections.sortDescending as _sortDescending
import kotlin.io.println as iprintln


/** @author Spheniscine */
fun main() { _writer.solve(); _writer.flush() }
fun PrintWriter.solve() {
//    val startTime = System.nanoTime()

    val numCases = 1//readInt()
    case@ for(case in 1..numCases) {
//        print("Case #$case: ")

        val n = readInt()
        val H = readIntArray(n+n)
        H.sort()

        fft = ModFFT(n)
        val polys = PriorityQueue<Poly>(compareBy { it.deg })

        ArrayModCombinatorics(n+n).run {
            val inv = ModIntArray(n+n + 1)
            for(i in 1..inv.lastIndex) { inv[i] = invf[i] * factorial[i-1] }

            H.forEachRun { _, a ->
                val P = ModIntArray(a/2+1)
                P[0] = ModInt(1)
                for(i in 0 until P.lastIndex) {
                    P[i+1] = P[i] * C(a-i-i, 2) * inv[i+1]
                }

                polys.add(Poly(P))
            }

            while(polys.size > 1) {
                val a = polys.remove()
                val b = polys.remove()
                a.mul(b)
                polys.add(a)
            }

            val P = polys.element().also { it.dft = false }.arr
            var ans = ModInt(0)

            val halfPow = invf[2].powArray(n)

            for(i in 0 .. min(n, P.lastIndex)) {
                val m = n - i
                val r = P[i] * factorial[m+m] * invf[m] * halfPow[m]
                if(i and 1 == 0) ans += r else ans -= r
            }

            println(ans.int)
        }

    }

//    iprintln("Time: ${(System.nanoTime() - startTime) / 1000000} ms")
}

inline fun IntArray.forEachRun(yield: (item: Int, num: Int) -> Unit) {
    if(isEmpty()) return
    var last = this[0]
    var num = 1

    for(i in 1 until size) {
        if(last == this[i]) num++
        else {
            yield(last, num)
            last = this[i]
            num = 1
        }
    }
    yield(last, num)
}

class ArrayModCombinatorics(val maxn: Int) {
    val factorial = ModIntArray(maxn + 1).also {
        it[0] = ModInt(1)
        for(i in 1 .. maxn) {
            it[i] = it[i-1] * i
        }
    }

    val invf = ModIntArray(maxn + 1).also {
        it[maxn] = factorial[maxn].inv_unmemoized()
        for(i in maxn downTo 1) {
            it[i-1] = it[i] * i
        }
    }

    fun P(n: Int, k: Int) = if(k > n) ModInt(0) else factorial[n] * invf[n-k]
    fun C(n: Int, k: Int) = if(k > n) ModInt(0) else factorial[n] * invf[k] * invf[n-k]

    // multi-choose, be sure to adjust maxn accordingly
    fun M(n: Int, k: Int) = if(k == 0) ModInt(1) else C(n + k - 1, k)
}

lateinit var fft: ModFFT
class Poly(var arr: ModIntArray, var deg: Int = arr.lastIndex) {
    var dft = false
        set(value) {
            if(field == value) return
            field = value
            fft.inPlace(arr, !value)
        }

    fun mul(b: Poly) {
        val arrSize = fft.getArrSize(deg + b.deg)
        if(arrSize != arr.size) {
            dft = false
            arr = arr.copyOf(arrSize)
        }
        if(arrSize != b.arr.size) {
            b.dft = false
            b.arr = b.arr.copyOf(arrSize)
        }

        dft = true
        b.dft = true
        for(i in arr.indices) arr[i] *= b.arr[i]
        deg += b.deg
    }
}

inline val PRIMITIVE_ROOT get() = ModInt(3) // for 998244353
class ModFFT(maxDeg: Int) {
    val n = getArrSize(maxDeg)
    private val ninv = ModInt(n).inv_unmemoized()

    fun getArrSize(deg: Int) = Integer.highestOneBit(deg shl 1 or 1)
    // prepares an array of suitable size for inPlace
    fun prepare(arr: ModIntArray, deg: Int) = arr.copyOf(getArrSize(deg))

    // w[1] represents n-th root of unity e^(i * tau/n), w[k] = w[1]^k
    val w = PRIMITIVE_ROOT.pow(TOTIENT / n).powArray(n-1)

    // rev[i] is reversal of bits in i
    private val rev = IntArray(n).also { rev ->
        var bit = 1
        var rbit = n shr 1
        while(bit < n) {
            for(i in 0 until bit) {
                rev[i or bit] = rbit or rev[i]
            }
            bit = bit shl 1
            rbit = rbit shr 1
        }
    }

    operator fun invoke(a: ModIntArray, invert: Boolean): ModIntArray {
        val res = a.copyOf(n)
        inPlace(res, invert)
        return res
    }

    // in-place FFT. Precondition: a.size <= n, and a.size is a power of 2
    fun inPlace(a: ModIntArray, invert: Boolean) {
        val sz = a.size
        if(sz <= 1) return
        require(sz <= n && sz and -sz == sz) { "Array size $sz must be less than $n and a power of 2" }
        val st = n / sz
        for(i in 0 until sz) {
            val j = rev[i * st]
            if(i < j) a[i] = a[j].also { a[j] = a[i] }
        }

        var len = 2
        var ang = n shr 1 // = n / len, representing the angle tau / len
        if(invert) ang = -ang
        while(len <= sz) {
            var i = 0
            val h = len shr 1
            while(i < sz) {
                var k = 0
                for(j in i until i+h) {
                    val u = a[j]
                    val v = a[j+h] * w[k]
                    a[j] = u + v
                    a[j+h] = u - v
                    k = k + ang and n-1
                }
                i += len
            }

            len = len shl 1
            ang = ang shr 1
        }

        if(invert) {
            val szinv = ninv * st
            for(i in 0 until sz) a[i] *= szinv
        }
    }

    fun mul(a: ModIntArray, b: ModIntArray): ModIntArray {
        val deg = a.lastIndex + b.lastIndex
        require(deg < n) { "FFT capacity $n too small for polynomial of degree $deg" }
        val a1 = prepare(a, deg)
        val b1 = prepare(b, deg)
        inPlace(a1, false)
        inPlace(b1, false)
        for(i in a1.indices) a1[i] *= b1[i]
        inPlace(a1, true)
        return a1//.copyOf(deg+1)
    }

    fun sq(a: ModIntArray): ModIntArray {
        val deg = a.lastIndex * 2
        require(deg < n) { "FFT capacity $n too small for polynomial of degree $deg" }
        val a1 = prepare(a, deg)
        inPlace(a1, false)
        for(i in a1.indices) a1[i] *= a1[i]
        inPlace(a1, true)
        return a1//.copyOf(deg+1)
    }
}


const val BILLION7 = 1e9.toInt() + 7
const val MOD = 998244353
const val TOTIENT = MOD - 1 // assumes MOD is prime

infix fun Int.modulo(mod: Int): Int = (this % mod).let { (it shr Int.SIZE_BITS - 1 and mod) + it }
infix fun Long.modulo(mod: Long) = (this % mod).let { (it shr Long.SIZE_BITS - 1 and mod) + it }
infix fun Long.modulo(mod: Int) = modulo(mod.toLong()).toInt()

fun Int.mulMod(other: Int, mod: Int) = toLong() * other modulo mod

fun Int.powMod(exponent: Long, mod: Int): Int {
    if(exponent < 0) error("Inverse not implemented")
    var res = 1L
    var e = exponent
    var b = modulo(mod).toLong()

    while(e > 0) {
        if(e and 1 == 1L) {
            res = res * b % mod
        }
        e = e shr 1
        b = b * b % mod
    }
    return res.toInt()
}
fun Int.powMod(exponent: Int, mod: Int) = powMod(exponent.toLong(), mod)
fun Int.modPowArray(n: Int, mod: Int): IntArray {
    val res = IntArray(n+1)
    res[0] = 1
    for(i in 1..n) res[i] = mulMod(res[i-1], mod)
    return res
}

inline fun Int.toModInt() = ModInt(this modulo MOD)
inline fun Long.toModInt() = ModInt(this modulo MOD)

/** note: Only use constructor for int within modulo range, otherwise use toModInt **/
inline class ModInt(val int: Int) {
    companion object {
        /** can't seem to make these private or inlined without causing compiler issues */
        @JvmField val _invMemo = HashMap<ModInt, ModInt>()
        fun _invMemoized(m: ModInt) = _invMemo.getOrPut(m) { m.inv_unmemoized() }
    }

    // normalizes an integer that's within range [-MOD, MOD) without branching
    private inline fun normalize(int: Int) = ModInt((int shr Int.SIZE_BITS - 1 and MOD) + int)

    operator fun plus(other: ModInt) = normalize(int + other.int - MOD) // overflow-safe even if MOD >= 2^30
    inline operator fun plus(other: Int) = plus(other.toModInt())
    operator fun inc() = normalize(int + (1 - MOD))

    operator fun minus(other: ModInt) = normalize(int - other.int)
    inline operator fun minus(other: Int) = minus(other.toModInt())
    operator fun dec() = normalize(int - 1)
    operator fun unaryMinus() = normalize(-int)

    operator fun times(other: ModInt) = ModInt((int.toLong() * other.int % MOD).toInt())
    inline operator fun times(other: Int) = ModInt(int.mulMod(other, MOD))

    fun pow(exponent: Int): ModInt {
        val e = if(exponent < 0) {
            require(int != 0) { "Can't invert/divide by 0" }
            exponent modulo TOTIENT
        } else exponent
        return ModInt(int.powMod(e, MOD))
    }

    fun pow(exponent: Long) = if(int == 0) when {
        exponent > 0 -> this
        exponent == 0L -> ModInt(1)
        else -> error("Can't invert/divide by 0")
    } else pow(exponent modulo TOTIENT)

    inline fun inverse() = inv_memoized() /** NOTE: Change if necessary */

    fun inv_unmemoized(): ModInt {
        require(int != 0) { "Can't invert/divide by 0" }
        return pow(TOTIENT - 1)
    }
    inline fun inv_memoized() = _invMemoized(this)

    operator fun div(other: ModInt) = times(other.inverse())
    inline operator fun div(other: Int) = div(other.toModInt())

    override inline fun toString() = int.toString()
}

inline operator fun Int.plus(modInt: ModInt) = modInt + this
inline operator fun Int.minus(modInt: ModInt) = toModInt() - modInt
inline operator fun Int.times(modInt: ModInt) = modInt * this
inline operator fun Int.div(modInt: ModInt) = modInt.inverse() * this

inline class ModIntArray(val intArray: IntArray): Collection<ModInt> {
    inline operator fun get(i: Int) = ModInt(intArray[i])
    inline operator fun set(i: Int, v: ModInt) { intArray[i] = v.int }

    override inline val size: Int get() = intArray.size
    inline val lastIndex get() = intArray.lastIndex
    inline val indices get() = intArray.indices

    override inline fun contains(element: ModInt): Boolean = element.int in intArray

    override fun containsAll(elements: Collection<ModInt>): Boolean = elements.all(::contains)

    override inline fun isEmpty(): Boolean = intArray.isEmpty()

    override fun iterator(): Iterator<ModInt> = object: Iterator<ModInt> {
        var index = 0
        override fun hasNext(): Boolean = index < size
        override fun next(): ModInt = get(index++)
    }

    fun copyOf(newSize: Int) = ModIntArray(intArray.copyOf(newSize))
    fun copyOf() = copyOf(size)
}
fun ModIntArray.copyInto(destination: ModIntArray, destinationOffset: Int = 0, startIndex: Int = 0, endIndex: Int = size) =
    ModIntArray(intArray.copyInto(destination.intArray, destinationOffset, startIndex, endIndex))
inline fun ModIntArray(size: Int) = ModIntArray(IntArray(size))
inline fun ModIntArray(size: Int, init: (Int) -> ModInt) = ModIntArray(IntArray(size) { init(it).int })

fun ModInt.powArray(n: Int) = ModIntArray(int.modPowArray(n, MOD))

inline fun ModIntArray.first() = get(0)
inline fun ModIntArray.last() = get(lastIndex)
inline fun ModIntArray.joinToString(separator: CharSequence) = intArray.joinToString(separator)
inline fun <R> ModIntArray.fold(init: R, op: (acc: R, ModInt) -> R) = intArray.fold(init) { acc, i -> op(acc, ModInt(i)) }
inline fun <R> ModIntArray.foldRight(init: R, op: (ModInt, acc: R) -> R) = intArray.foldRight(init) { i, acc -> op(ModInt(i), acc) }
fun ModIntArray.sum() = fold(ModInt(0), ModInt::plus)
fun ModIntArray.product() = fold(ModInt(1), ModInt::times)

inline fun <T> Iterable<T>.sumByModInt(func: (T) -> ModInt) = fold(ModInt(0)) { acc, t -> acc + func(t) }
inline fun <T> Iterable<T>.productByModInt(func: (T) -> ModInt) = fold(ModInt(1)) { acc, t -> acc * func(t) }
inline fun <T> Sequence<T>.sumByModInt(func: (T) -> ModInt) = fold(ModInt(0)) { acc, t -> acc + func(t) }
inline fun <T> Sequence<T>.productByModInt(func: (T) -> ModInt) = fold(ModInt(1)) { acc, t -> acc * func(t) }
inline fun <T> Array<T>.sumByModInt(func: (T) -> ModInt) = fold(ModInt(0)) { acc, t -> acc + func(t) }
inline fun <T> Array<T>.productByModInt(func: (T) -> ModInt) = fold(ModInt(1)) { acc, t -> acc * func(t) }
fun Iterable<ModInt>.sum() = sumByModInt { it }
fun Sequence<ModInt>.sum() = sumByModInt { it }
fun Iterable<ModInt>.product() = productByModInt { it }
fun Sequence<ModInt>.product() = productByModInt { it }
fun Collection<ModInt>.toModIntArray() = ModIntArray(size).also { var i = 0; for(e in this) { it[i++] = e } }



/** IO */
//@JvmField val INPUT = File("input.txt").inputStream()
//@JvmField val OUTPUT = File("output.txt").outputStream()
@JvmField val INPUT = System.`in`
@JvmField val OUTPUT = System.out

const val _BUFFER_SIZE = 1 shl 16
@JvmField val _buffer = ByteArray(_BUFFER_SIZE)
@JvmField var _bufferPt = 0
@JvmField var _bytesRead = 0

tailrec fun readChar(): Char {
    if(_bufferPt == _bytesRead) {
        _bufferPt = 0
        _bytesRead = INPUT.read(_buffer, 0, _BUFFER_SIZE)
    }
    return if(_bytesRead < 0) Char.MIN_VALUE
    else {
        val c = _buffer[_bufferPt++].toChar()
        if (c == '\r') readChar()
        else c
    }
}

fun readLine(): String? {
    var c = readChar()
    return if(c == Char.MIN_VALUE) null
    else buildString {
        while(c != '\n' && c != Char.MIN_VALUE) {
            append(c)
            c = readChar()
        }
    }
}
fun readLn() = readLine()!!

fun read() = buildString {
    var c = readChar()
    while(c <= ' ') {
        if(c == Char.MIN_VALUE) return@buildString
        c = readChar()
    }
    do {
        append(c)
        c = readChar()
    } while(c > ' ')
}
fun readInt() = read().toInt()
fun readDouble() = read().toDouble()
fun readLong() = read().toLong()
fun readStrings(n: Int) = List(n) { read() }
fun readLines(n: Int) = List(n) { readLn() }
fun readInts(n: Int) = List(n) { read().toInt() }
fun readIntArray(n: Int) = IntArray(n) { read().toInt() }
fun readDoubles(n: Int) = List(n) { read().toDouble() }
fun readDoubleArray(n: Int) = DoubleArray(n) { read().toDouble() }
fun readLongs(n: Int) = List(n) { read().toLong() }
fun readLongArray(n: Int) = LongArray(n) { read().toLong() }

@JvmField val _writer = PrintWriter(OUTPUT, false)

/** shuffles and sort overrides to avoid quicksort attacks */
private inline fun <T> _shuffle(rnd: Random, get: (Int) -> T, set: (Int, T) -> Unit, size: Int) {
    // Fisher-Yates shuffle algorithm
    for (i in size - 1 downTo 1) {
        val j = rnd.nextInt(i + 1)
        val temp = get(i)
        set(i, get(j))
        set(j, temp)
    }
}

@JvmField var _random: Random? = null
val random get() = _random ?: Random(0x594E215C123 * System.nanoTime()).also { _random = it }

fun IntArray.shuffle(rnd: Random = random) = _shuffle(rnd, ::get, ::set, size)
fun IntArray.sort() { shuffle(); _sort() }
fun IntArray.sortDescending() { shuffle(); _sortDescending() }

fun LongArray.shuffle(rnd: Random = random) = _shuffle(rnd, ::get, ::set, size)
fun LongArray.sort() { shuffle(); _sort() }
fun LongArray.sortDescending() { shuffle(); _sortDescending() }

fun DoubleArray.shuffle(rnd: Random = random) = _shuffle(rnd, ::get, ::set, size)
fun DoubleArray.sort() { shuffle(); _sort() }
fun DoubleArray.sortDescending() { shuffle(); _sortDescending() }

fun CharArray.shuffle(rnd: Random = random) = _shuffle(rnd, ::get, ::set, size)
inline fun CharArray.sort() { _sort() }
inline fun CharArray.sortDescending() { _sortDescending() }

inline fun <T: Comparable<T>> Array<out T>.sort() = _sort()
inline fun <T: Comparable<T>> Array<out T>.sortDescending() = _sortDescending()
inline fun <T: Comparable<T>> MutableList<out T>.sort() = _sort()
inline fun <T: Comparable<T>> MutableList<out T>.sortDescending() = _sortDescending()

// import preserving junk function
@Suppress("NonAsciiCharacters") fun 雪花飄飄北風嘯嘯天地一片蒼茫() { iprintln(max(1, 2)) }

/** additional commons */
inline fun <T> Iterable<T>.sumByLong(func: (T) -> Long) = fold(0L) { acc, t -> acc + func(t) }
inline fun <T> Sequence<T>.sumByLong(func: (T) -> Long) = fold(0L) { acc, t -> acc + func(t) }
inline fun <T> Array<T>.sumByLong(func: (T) -> Long) = fold(0L) { acc, t -> acc + func(t) }
fun IntArray.sumLong() = fold(0L, Long::plus)