Recursive and iterative implementations of basic tree traversals in python 3

tree_traversal.py

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

def buildTree(arr):
    nodes = [TreeNode(val) for val in arr]
    for i in range(len(nodes)):
        left, right = (i+1)*2-1, (i+1)*2
        if left < len(arr):
            nodes[i].left = nodes[left]
        if right < len(arr):
            nodes[i].right = nodes[right]
    return nodes[0]

def preorderRecursive(node):
    if node is None:
        return []
    return [node.val] + preorderRecursive(node.left) + preorderRecursive(node.right)

def preorderIterative(node):
    res, stack = [], [(1, node)] # flag, node
    while len(stack) > 0:
        flag, node = stack.pop()
        if node is not None:
            if flag == 0:
                res.append(node.val)
            else:
                stack.extend([(1, node.right), (1, node.left), (0, node)])
    return res

def inorderRecursive(node):
    if node is None:
        return []
    return inorderRecursive(node.left) + [node.val] + inorderRecursive(node.right)

def inorderIterative(node):
    res, stack = [], [(1, node)] # flag, node
    while len(stack) > 0:
        flag, node = stack.pop()
        if node is not None:
            if flag == 0:
                res.append(node.val)
            else:
                stack.extend([(1, node.right), (0, node), (1, node.left)])
    return res

def postorderRecursive(node):
    if node is None:
        return []
    return postorderRecursive(node.left) + postorderRecursive(node.right) + [node.val]

def postorderIterative(node):
    res, stack = [], [(1, node)] # flag, node
    while len(stack) > 0:
        flag, node = stack.pop()
        if node is not None:
            if flag == 0:
                res.append(node.val)
            else:
                stack.extend([(0, node), (1, node.right), (1, node.left)])
    return res

if __name__ == "__main__":
    head = buildTree([1,2,3,4,5,6,7])
    print(preorderRecursive(head)) # [1, 2, 4, 5, 3, 6, 7]
    print(preorderIterative(head)) # [1, 2, 4, 5, 3, 6, 7]
    print(inorderRecursive(head)) # [4, 2, 5, 1, 6, 3, 7]
    print(inorderIterative(head)) # [4, 2, 5, 1, 6, 3, 7]
    print(postorderRecursive(head)) # [4, 5, 2, 6, 7, 3, 1]
    print(postorderIterative(head)) # [4, 5, 2, 6, 7, 3, 1]