https://leetcode.com/problems/unique-binary-search-trees/ Swift solution

UniqueBinaryTrees.swift

var cache = [Int:Int]()
func numTrees(n:Int) -> Int{
    if n < 2{
        return 1
    }
    if n == 2{
        return 2
    }
    
    if let value = cache[n]{
        return value
    }
    var count = 0
    for m in 0..<n {
        count += numTrees(m) * numTrees(n - m - 1)
    }
    cache[n] = count
    return count
}