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:

1. The tree will have between 1 and 100 nodes.

思路：BFS層序遍歷

# Definition for a binary tree node.
class TreeNode(object):
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

class Solution(object):
    def isCompleteTree(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        q,qq=[root],[]
        depth=1
        end=0
        while q:
            while q:
                s=q.pop(0)
                if not s: continue
                qq.append(s.left)
                qq.append(s.right)
            
            q,qq=qq,q
            if None in q:
                none_start = q.index(None)
                for i in range(none_start, len(q)):
                    if q[i]: return False
                q=q[:none_start]
                if not q: break
                if len(q)!=2**depth: end+=1
            
            depth+=1
        return end<=1