Python 中的 PreOrder 树遍历

Question

I am trying to write a function that takes a node as an argument and returns a string of the PreOrder traversal of that binary tree.

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

root = Node(1)
root.left = Node(2)
root.left.right = Node(4)
root.left.left = Node(5)
root.right = Node(3) 

def PreOrder(node): 

    path = ''

    if node:
        #print(node.value)
        path += (str(node.value) + "_")
        PreOrder(node.left)
        PreOrder(node.right)

    return path

print(PreOrder(root))

output: 1_

Thanks: )

Answer 1

I think you need to add PreOrder(node.left) and PreOrder(node.right) to the path.

So the code would look like:

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

#                   1
#               2       3
#           5       4

def PreOrder(node): 

    path = ''

    if node:
        path += (str(node.value) + "_")
        path += PreOrder(node.left) 
        path += PreOrder(node.right)

    return path

print(PreOrder(root))

# prints 1_2_5_ 4_  3_

Answer 2

Check this link . This is a reference link from where you can see more traversal techniques. I am providing code below for Pre-Order traversal. Check this reference and compare your code with that where you need to correct.

class Node:

    def __init__(self, data):

        self.left = None
        self.right = None
        self.data = data
# Insert Node
    def insert(self, data):

        if self.data:
            if data < self.data:
                if self.left is None:
                    self.left = Node(data)
                else:
                    self.left.insert(data)
            elif data > self.data:
                if self.right is None:
                    self.right = Node(data)
                else:
                    self.right.insert(data)
        else:
            self.data = data

# Print the Tree
    def PrintTree(self):
        if self.left:
            self.left.PrintTree()
        print( self.data),
        if self.right:
            self.right.PrintTree()

# Preorder traversal
# Root -> Left ->Right
    def PreorderTraversal(self, root):
        res = []
        if root:
            res.append(root.data)
            res = res + self.PreorderTraversal(root.left)
            res = res + self.PreorderTraversal(root.right)
        return res

root = Node(1)
root.insert(2)
root.insert(3)
root.insert(4)
root.insert(5)
print(root.PreorderTraversal(root))