Solution.kt

class Memo(val f: Memo.(n: Int) -> Int) {
    val hash = HashMap<Int, Int>()
    operator fun invoke(n: Int) : Int {
        if (!hash.contains(n)) { hash[n] = this.f(n) }
        return hash[n] ?: throw RuntimeException("Souldn't happen")
    }
}

class Solution {
    fun climbStairs(n: Int): Int {
        val search = Memo {
            when (it) {
                1 -> 1
                2 -> 2
                else -> this(it - 1) + this(it - 2)
            }
        }
        
        return search(n)
    }
}