Решение

class MyStack() {
    private var a = IntArray(16)
    private var n = 0
    fun push(x: Int) {
        if (n == a.size) a = a.copyOf(a.size shl 1)
        a[n++] = x
    }
    fun pop(): Int = a[--n]
    fun top(): Int = a[n - 1]
    fun empty(): Boolean = n == 0
}