Python 在空二叉树中打印特定字符 Output

Question

I have a binary tree generated as follows

class Node:
    def __init__(self,data):
        # Left Child
        self.left = None
        # Right Child
        self.right = None
        # Node Value
        self.data = data

    def insert(self, data):
    # Compare the new value with the parent node
        if self.data:
            # Less than or equal goes to the Left
            if data <= self.data:
                if self.left is None:
                    self.left = Node(data)
                else:
                    self.left.insert(data)
            # Greater than goes to the right
            elif data > self.data:
                if self.right is None:
                    self.right = Node(data)
                else:
                    self.right.insert(data)
        else:
            self.data = data

Now suppose we populate the binary tree with the following

Numbers = [4, 7, 9, 1 , 3]

for index in range(len(Numbers)):
    if index == 0:
        root = Node(Numbers[index])
    else:
         root.insert(Numbers[index])

Which generates the following binary tree:

      4
     / \
   1     7
  / \    / \
 .   3  .   9

I have written the following to print the binary tree:

def PrintTree(self, root):
    thislevel = [root]

    while thislevel:
        nextlevel = list()
        Curr_Level = ""
        for n in thislevel:
            Curr_Level += " " + str(n.data)

            if n.left: 
                nextlevel.append(n.left)
            if n.right: 
                nextlevel.append(n.right)

        print(Curr_Level)
        thislevel = nextlevel

root.PrintTree(root=root)

which generates the following output

4
1 7
3 9

However, I would like the code print the empty entries with an "@", ie I want my code to output the following:

4
1 7
@ 3 @ 9

How can I go about adjusting my PrintTree function to achieve this?

Answer 1

Run a modified version of BFS. More specifically, run breadth first search (BFS) starting from the root node. This will assign a level to each node. During BFS, when a current node has a missing child, add an @ node in its place to the FIFO queue. Print a node each time it is removed from the queue. Print in a new line each time a node with a new level is removed from the queue.