Problem:
Score : 500 points

Problem Statement

N players will participate in a tennis tournament. We will call them Player 1, Player 2, \ldots, Player N.

The tournament is round-robin format, and there will be N(N-1)/2 matches in total.
Is it possible to schedule these matches so that all of the following conditions are satisfied? If the answer is yes, also find the minimum number of days required.

Each player plays at most one matches in a day.

Each player i (1 \leq i \leq N) plays one match against Player A_{i, 1}, A_{i, 2}, \ldots, A_{i, N-1} in this order.

Constraints

3 \leq N \leq 1000

1 \leq A_{i, j} \leq N

A_{i, j} \neq i

A_{i, 1}, A_{i, 2}, \ldots, A_{i, N-1} are all different.

Input

Input is given from Standard Input in the following format:

N
A_{1, 1} A_{1, 2} \ldots A_{1, N-1}
A_{2, 1} A_{2, 2} \ldots A_{2, N-1}
:
A_{N, 1} A_{N, 2} \ldots A_{N, N-1}

Output

If it is possible to schedule all the matches so that all of the conditions are satisfied, print the minimum number of days required; if it is impossible, print -1.

Sample Input 1

3
2 3
1 3
1 2

Sample Output 1

3

All the conditions can be satisfied if the matches are scheduled for three days as follows:

Day 1: Player 1 vs Player 2

Day 2: Player 1 vs Player 3

Day 3: Player 2 vs Player 3

This is the minimum number of days required.

Sample Input 2

4
2 3 4
1 3 4
4 1 2
3 1 2

Sample Output 2

4

All the conditions can be satisfied if the matches are scheduled for four days as follows:

Day 1: Player 1 vs Player 2, Player 3 vs Player 4

Day 2: Player 1 vs Player 3

Day 3: Player 1 vs Player 4, Player 2 vs Player 3

Day 4: Player 2 vs Player 4

This is the minimum number of days required.

Sample Input 3

3
2 3
3 1
1 2

Sample Output 3

-1

Any scheduling of the matches violates some condition.

Solution in Kotlin (1.0.0) (judge verdict: Time Limit Exceeded)

import java.io.PrintWriter
import java.util.*
import java.io.IOException
import java.util.NoSuchElementException

fun main(args: Array<String>) {
    val out = PrintWriter(System.out)
    val sc = FastScanner()
    val n = sc.nextInt()
    val a = mutableListOf<MutableList<Int>>()
    for (i in 0 until n) {
        a.add(mutableListOf())
        for (j in 0 until n - 1) {
            a[i].add(sc.next().toInt())
        }
    }
    out.println(problem139e(n, a))
    out.flush()
}

fun problem139e(n: Int, a: MutableList<MutableList<Int>>): Int {
    var count = 0
    val list = mutableListOf<HashSet<Int>>()
    for (i in 0 until a.size) {
        list.add(hashSetOf())
    }
    var isChanged = false
    while (!a.all { it.isEmpty() }) {
        count++
        val used = hashSetOf<Int>()
        for (i in 0 until a.size) {
            if (a[i].isEmpty()) continue
            if (!used.contains(i)) {
                val player = a[i][0] - 1
                if (!used.contains(player) && a[player][0] - 1 == i && !list[i].contains(player)) {
                    a[i].removeAt(0)
                    a[player].removeAt(0)
                    used.add(i)
                    used.add(player)
                    list[i].add(player)
                    isChanged = true
                }
            }
        }
        if (!isChanged) {
            return -1
        }
        isChanged = false
    }
    return count
}

class FastScanner {
    private val `in` = System.`in`
    private val buffer = ByteArray(1024)
    private var ptr = 0
    private var buflen = 0
    private fun hasNextByte(): Boolean {
        if (ptr < buflen) {
            return true
        } else {
            ptr = 0
            try {
                buflen = `in`.read(buffer)
            } catch (e: IOException) {
                e.printStackTrace()
            }

            if (buflen <= 0) {
                return false
            }
        }
        return true
    }

    private fun readByte(): Int {
        return if (hasNextByte()) buffer[ptr++].toInt() else -1
    }

    private fun isPrintableChar(c: Int): Boolean {
        return 33 <= c && c <= 126
    }

    operator fun hasNext(): Boolean {
        while (hasNextByte() && !isPrintableChar(buffer[ptr].toInt())) ptr++
        return hasNextByte()
    }

    operator fun next(): String {
        if (!hasNext()) throw NoSuchElementException()
        val sb = StringBuilder()
        var b = readByte()
        while (isPrintableChar(b)) {
            sb.appendCodePoint(b)
            b = readByte()
        }
        return sb.toString()
    }

    fun nextLong(): Long {
        if (!hasNext()) throw NoSuchElementException()
        var n: Long = 0
        var minus = false
        var b = readByte()
        if (b == '-'.toInt()) {
            minus = true
            b = readByte()
        }
        if (b < '0'.toInt() || '9'.toInt() < b) {
            throw NumberFormatException()
        }
        while (true) {
            if ('0'.toInt() <= b && b <= '9'.toInt()) {
                n *= 10
                n += (b - '0'.toInt()).toLong()
            } else return if (b == -1 || !isPrintableChar(b)) {
                if (minus) -n else n
            } else {
                throw NumberFormatException()
            }
            b = readByte()
        }
    }

    fun nextInt(): Int {
        val nl = nextLong()
        if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) throw NumberFormatException()
        return nl.toInt()
    }

    fun nextDouble(): Double {
        return java.lang.Double.parseDouble(next())
    }
}