## Description

Given a binary tree, determine if it is a *complete binary tree*.

**Definition of a complete binary tree from Wikipedia:**
In a complete binary tree 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 2h nodes inclusive at the last level h.

**Example 1:**

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

**Note:**

- The tree will have between 1 and 100 nodes.

## Solutions

### 1. BFS-Travel by level

```
# Time: O(n)
# Space: O(n)
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution:
def isCompleteTree(self, root: TreeNode) -> bool:
if not root:
return True
queue = collections.deque()
queue.append(root)
is_1st_none = False
while queue:
node = queue.popleft()
if not node:
is_1st_none = True
else:
if is_1st_none:
return False
queue.append(node.left)
queue.append(node.right)
return True
# 119/119 cases passed (28 ms)
# Your runtime beats 93.74 % of python3 submissions
# Your memory usage beats 100 % of python3 submissions (12.8 MB)
```