958.Check Completeness of a Binary Tree
===

###### tags: `Medium`,`Tree`,`BFS`

[958. Check Completeness of a Binary Tree](https://leetcode.com/problems/check-completeness-of-a-binary-tree/)

### 題目描述

Given the `root` of a binary tree, determine if it is a *complete binary tree*.

In a [complete binary tree](https://en.wikipedia.org/wiki/Binary_tree#Types_of_binary_trees), every level, except possibly the last, is completely filled, and all nodes in the last level are as far left as possible. It can have between 1 and 2^h^ nodes inclusive at the last level `h`.

### 範例

**Example 1:**


Input: root = [1,2,3,4,5,6]
Output: true
Explanation: Every level before the last is full (ie. levels with node-values {1} and {2, 3}), and all nodes in the last level ({4, 5, 6}) are as far left as possible.

**Example 2:**


Input: root = [1,2,3,4,5,null,7]
Output: false
Explanation: The node with value 7 isn't as far left as possible.

**Constraints**:

* The number of nodes in the tree is in the range `[1, 100]`.
* 1 <= `Node.val` <= 1000

### 解答
#### Python

class Solution:
    def isCompleteTree(self, root: Optional[TreeNode]) -> bool:
        dq = deque([root])
        null_node_found = False

        while dq:
            node = dq.popleft()

            if not node:
                null_node_found = True
            else:
                if null_node_found:
                    return False

                dq.append(node.left)
                dq.append(node.right)

        return True

> [name=Ron Chen][time= Wed, Mar 15, 2023]

### Reference

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