Leetcode #250: Count Univalue Subtrees

func countUnivalSubtrees(_ root: TreeNode?) -> Int {
    if root == nil {
        return 0
    }
    if root?.left == nil && root?.right == nil {
        return 1
    }
    
    var count = countUnivalSubtrees(root?.left) + countUnivalSubtrees(root?.right)
    if isUnival(root?.left, root!.val) && isUnival(root?.right, root!.val) {
        count += 1
    }
    return count
}

private func isUnival(_ node: TreeNode?, _ val: Int) -> Bool {
    if node == nil {
        return true
    }
    if node!.val != val {
        return false
    }
    return isUnival(node?.left, val) && isUnival(node?.right, val)
}