Boundary of Binary Tree — Python

# Time:  O(n)
# Space: O(h)

class Solution(object):
    def boundaryOfBinaryTree(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        def leftBoundary(root, nodes):
            if not root or (not root.left and not root.right):
                return
            nodes.append(root.val)
            if not root.left:
                leftBoundary(root.right, nodes)
            else:
                leftBoundary(root.left, nodes)

        def rightBoundary(root, nodes):
            if not root or (not root.left and not root.right):
                return
            if not root.right:
                rightBoundary(root.left, nodes)
            else:
                rightBoundary(root.right, nodes)
            nodes.append(root.val)

        def leaves(root, nodes):
            if not root:
                return
            if not root.left and not root.right:
                nodes.append(root.val)
                return
            leaves(root.left, nodes)
            leaves(root.right, nodes)

        if not root:
            return []

        nodes = [root.val]
        leftBoundary(root.left, nodes)
        leaves(root.left, nodes)
        leaves(root.right, nodes)
        rightBoundary(root.right, nodes)
        return nodes