遍历二叉树:在python中无法得到正确的输出

Question

我正在输入预排序遍历，输出只是'1-';我已经浏览了这个网站上的其他几个线程，但似乎找不到我的错误。任何提示或建议将不胜感激!

class Node(object):

def __init__(self, value):
self.number = value
self.left = None
self.right = None
class binarytree(object):

def __init__(self, root):
self.root = Node(root)
def print_traversal(self, direction, start):
if direction == 'pre_order':
return self.pre_order(start, '')

def pre_order(self, start, traversal):
if start:
traversal += (str(start.number) + '-' )
self.pre_order(start.left, traversal)
self.pre_order(start.right, traversal)
return traversal
# main function
tree1 = binarytree(1)
tree1.root.left = Node(2)
tree1.root.right = Node(3)
tree1.root.left.left = Node(4)
tree1.root.left.right = Node(5)
tree1.root.right.left = Node(6)
print(tree1.print_traversal('pre_order', tree1.root))```

Answer 1

traversal += (str(start.number) + '-' )

给局部变量traversal赋一个新值;不修改traversal中已经存在的字符串。因此，在递归调用中所做的更改永远不会在它们所在的级别上看到。