Решение

class OrderedStream(n: Int) {
    private val stream = arrayOfNulls<String>(n + 1)
    private var ptr = 1
    fun insert(idKey: Int, value: String): List<String> {
        stream[idKey] = value
        val res = mutableListOf<String>()
        while (ptr < stream.size && stream[ptr] != null) {
            res.add(stream[ptr]!!)
            ptr++
        }
        return res
    }
}

/**
 * Your OrderedStream object will be instantiated and called as such:
 * var obj = OrderedStream(n)
 * var param_1 = obj.insert(idKey,value)
 */