LeetCode/kt/student-attendance-record-ii.kt

val MOD = 1000000007

class Solution {
    private fun createState(): MutableList<MutableList<Int>> = (0..1).map { mutableListOf(0, 0, 0) }.toMutableList()

    fun checkRecord(n: Int): Int {
        val initialState = createState()
        initialState[0][0] = 1

        val finalState =
            (1..n).fold(initialState) { state, _ ->
                val nextState = createState()

                (0..1).flatMap { absences -> (0..2).map { lates -> absences to lates } }.forEach { (absences, lates) ->
                    nextState[absences][0] = (nextState[absences][0] + state[absences][lates]) % MOD

                    if (absences < 1) {
                        nextState[absences + 1][0] = (nextState[absences + 1][0] + state[absences][lates]) % MOD
                    }

                    if (lates < 2) {
                        nextState[absences][lates + 1] = (nextState[absences][lates + 1] + state[absences][lates]) % MOD
                    }
                }

                return@fold nextState
            }

        return finalState.fold(0) { acc, it ->
            it.fold(acc) { acc, it ->
                (acc + it) % MOD
            }
        }
    }
}

fun main() {
    val s = Solution()

    check(s.checkRecord(2) == 8)
    check(s.checkRecord(1) == 3)
    check(s.checkRecord(10101) == 183236316)
}