I'm currently working on LeetCode problem 108. Convert Sorted Array to Binary Search Tree:

Given an integer array nums where the elements are sorted in ascending order, convert it to a height-balanced binary search tree.

A height-balanced binary tree is a binary tree in which the depth of the two subtrees of every node never differs by more than one.

My code seems to be working fine but I don't know how to display the value null instead of None in my list. I need to print the BST in level order traversal. I'm looking for advice, hints or suggestions.

Input:

[-10,-3,0,5,9]

My current output:

[0, -3, 9, -10, None, 5, None]

Accepted output:

[0,-3,9,-10,null,5]

Here is my code:

from queue import Queue
from typing import Optional

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def sortedArrayToBST(nums: [int]) -> Optional[TreeNode]:
    nbNodes = len(nums)

    if nbNodes == 1:
        root = TreeNode()
        root.val = nums[0]
        return root
    elif nbNodes == 0:
        root = TreeNode()
        root.val = None
        return root

    middle = int(nbNodes / 2)
    root = TreeNode()
    root.val = nums[middle]
    leftList = []
    rightList = []
    j = middle   1

    for i in range(middle):
        leftList.append(nums[i])

        if j != nbNodes:
            rightList.append(nums[j])
        j  = 1

    root.left = sortedArrayToBST(leftList)
    root.right = sortedArrayToBST(rightList)
    return root

def levelorder(root):
    if root==None:
        return
    Q=Queue()
    Q.put(root)
    level_order_list = []
    while(not Q.empty()):
        node=Q.get()
        if node==None:
            continue
        level_order_list.append(node.val)
        Q.put(node.left)
        Q.put(node.right)
    print(level_order_list)

if __name__ == "__main__":
    container = [-10,-3,0,5,9]
    levelorder(sortedArrayToBST(container))

CodePudding user response：

The problem is not related to null or None. LeetCode is a platform for taking code challenges in many different languages and they use JSON style to describe input/output, and in JSON notation None translates to null.

Not to worry about that. However, when you look at your output, there is a trailing None that should not be there. That means that you returned a BST that has a node with a None value. This should not happen.

The code that creates this None valued node is easy to identify... it is here:

    elif nbNodes == 0:
        root = TreeNode()
        root.val = None
        return root
        return

When you think of it, this cannot be right: the number of nodes to generate (nbNodes) is 0, yet your code creates 1 node -- and that is one too many! In this case you should just return None to indicate that the parent node has no child here.

So replace with:

    elif nbNodes == 0:
        return

This fixes the issue you mentioned, and your code will now pass all tests on LeetCode.

CodePudding user response：

This is kind of a weird requirement that has nothing to do with the apparent main point of the problem and I suspect it's a result of the description being copied over from one that's aimed at another language (like Javascript, which uses null instead of None).

You can, however, format your list however you like when you print it; here's an example where we print a list by joining each item with "," (instead of the default ", ") and replace None with "null":

>>> my_list = [0, -3, 9, -10, None, 5, None]
>>> print("["   ",".join("null" if i is None else str(i) for i in my_list)   "]")
[0,-3,9,-10,null,5,null]

Since JSON renders None as null, another option would be to dump the list to JSON and remove the " " characters:

>>> import json
>>> print(json.dumps(my_list).replace(' ', ''))
[0,-3,9,-10,null,5,null]