二叉树的前序遍历-python

def pre_order(root):
    if root == None:
        print("")
    stack = []
    stack.append(root)
    
    while len(stack):
        pop = stack.pop()
        print(pop.val)
        if pop.right != None:
            stack.append(pop.right)
        if pop.left != None:
            stack.append(pop.left)

前序：跟左右，先进右，后近进左