Boundary Traversal of a Binary Tree - Iterative Approach

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def isLeaf(node):
    return node.left is None and node.right is None

def addLeftBoundary(root, res):
    cur = root.left
    while cur:
        if not isLeaf(cur):
            res.append(cur.val)
        cur = cur.left if cur.left else cur.right

def addLeaves(root, res):
    if root is None:
        return
    if isLeaf(root):
        res.append(root.val)
    else:
        addLeaves(root.left, res)
        addLeaves(root.right, res)

def addRightBoundary(root, res):
    cur = root.right
    tmp = []
    while cur:
        if not isLeaf(cur):
            tmp.append(cur.val)
        cur = cur.right if cur.right else cur.left
    res.extend(tmp[::-1])

def boundaryTraversal(root):
    if not root:
        return []
    res = []
    if not isLeaf(root):
        res.append(root.val)
    addLeftBoundary(root, res)
    addLeaves(root, res)
    addRightBoundary(root, res)
    return res

if __name__ == '__main__':
    # Construct binary tree:
    #         1
    #        / \
    #       2   3
    #      / \   \
    #     4   5   6
    #            /
    #           7
    root = TreeNode(1, TreeNode(2, TreeNode(4), TreeNode(5)), TreeNode(3, None, TreeNode(6, TreeNode(7))))
    print(boundaryTraversal(root))