# Various iterative ways of traversing a tree.
def inorder_traversal(root):
    """
    :type root: TreeNode
    :rtype: List[int]
    """
    if not root:
      return []
    result = []
    stack = [root]
    while len(stack) > 0:
        curr_node = stack.pop()
        if curr_node.left:
            stack.append(curr_node)
            stack.append(curr_node.left)
            curr_node.left = None
        else:
            result.append(curr_node.val)
            if curr_node.right:
                stack.append(curr_node.right)
    return result

def preorder_traversal(root):
    """
    :type root: TreeNode
    :rtype: List[int]
    """
    if not root:
        return []
    result = []
    stack = [root]
    while len(stack) > 0:
        curr_node = stack.pop()
        result.append(curr_node.val)
        if curr_node.right:
            stack.append(curr_node.right)
        if curr_node.left:
            stack.append(curr_node.left)
    return result

def postorder_traversal(root):
    """
    :type root: TreeNode
    :rtype: List[int]
    """
    if not root:
        return []
    result = []
    stack = [root]
    while len(stack) > 0:
        curr_node = stack.pop()
        if curr_node.left:
            stack.append(curr_node)
            stack.append(curr_node.left)
            curr_node.left = None
        elif curr_node.right:
            stack.append(curr_node)
            stack.append(curr_node.right)
            curr_node.right = None
        else:
            result.append(curr_node.val)
    return result