545. Boundary of Binary Tree

class Solution(object):
    def dfs_leftmost(self, root, res):
        if not root or not root.left and not root.right:
            return

        res.append(root.val)
        if root.left:
            self.dfs_leftmost(root.left, res)
        else:
            self.dfs_leftmost(root.right, res)

    def dfs_leaves(self, root, res, exl_node):
        if not root:
            return

        self.dfs_leaves(root.left, res, exl_node)
        if root != exl_node and not root.left and not root.right:
            res.append(root.val)
        self.dfs_leaves(root.right, res, exl_node)

    def dfs_rightmost(self, root, res):
        if not root or not root.left and not root.right:
            return

        if root.right:
            self.dfs_rightmost(root.right, res)
        else:
            self.dfs_rightmost(root.left, res)
        res.append(root.val)


    def boundaryOfBinaryTree(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        res = []
        if not root:
            return res
        res.append(root.val)
        self.dfs_leftmost(root.left, res)
        self.dfs_leaves(root, res, root)
        self.dfs_rightmost(root.right, res)
        return res