// MARK: - Minimum Depth of Binary Tree

func minDepth(_ root: TreeNode?) -> Int {
    if root == nil {
        return 0
    }

    if root?.left != nil && root?.right != nil {
        return min(minDepth(root?.left), minDepth(root?.right)) + 1
    }

    if root?.left != nil {
        return minDepth(root?.left) + 1
    }

    return minDepth(root?.right) + 1
}




let root = TreeNode(2)
root.right = TreeNode(3)
root.right?.right = TreeNode(4)
root.right?.right?.right = TreeNode(5)
root.right?.right?.right?.right = TreeNode(6)

minDepth(root)