inorder.py

class Node:

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


def inorder(root):
    res = []
    if not root:
        return res
    stack = []
    while root or stack:
        while root:
            stack.append(root)
            root = root.left
        root = stack.pop()
        res.append(root.val)
        root = root.right
    return res

# Recursive Implementation
def inorder_rec(root, res=None):
    if root is None:
        return []
    if res is None: 
        res = []
    inorder_rec(root.left, res)
    res.append(root.val)
    inorder_rec(root.right, res)
    return res

if __name__ == '__main__':
    n1 = Node(100)
    n2 = Node(50)
    n3 = Node(150)
    n4 = Node(25)
    n5 = Node(75)
    n6 = Node(125)
    n7 = Node(175)
    n1.left, n1.right = n2, n3
    n2.left, n2.right = n4, n5
    n3.left, n3.right = n6, n7
    
    assert inorder(n1)     == [25, 50, 75, 100, 125, 150, 175]
    assert inorder_rec(n1) == [25, 50, 75, 100, 125, 150, 175]