Compute length of longest increasing subsequence in Swift

lis.swift

// Playground - noun: a place where people can play

let array = [3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5, 8, 9, 7, 9, 3, 2, 8, 4, 6, 2, 7]

// Computes the length of the longest increasing subsequence in the list of integers.
func lis(list: [Int]) -> Int {
	return lis(0, list, nil)
}

func lis(index: Int, list: [Int], largestValue: Int?) -> Int {
	if index >= list.count {
		return 0
	}
	
	let currentValue = list[index]
	
	if currentValue <= largestValue {
		return lis(index + 1, list, largestValue)
	}
	
	return max(lis(index + 1, list, largestValue == nil ? currentValue : largestValue), lis(index + 1, list, currentValue) + 1)
}

lis(array)