Judge whether it is a complete binary tree

# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
#
#  The class name in the code 、 Method name 、 The parameter name has been specified , Do not modify , Return the value specified by the method directly 
#
# 
# @param root TreeNode class  
# @return bool Boolean type 
#
class Solution:
    def isCompleteTree(self , root: TreeNode) -> bool:
        # write code here
        #  Judging a complete binary tree ： Sequence traversal , Set up flag  Judge whether the current node exists 
        
        if not root:
            return 
        node_lst = []
        node_lst.append(root)
        flag = False
        
        while node_lst:
            layer_node = []
            while node_lst:
                cur = node_lst.pop(0)
                if not cur:
                    flag =  True
                else:
                    if flag:
                        return False
                    layer_node.append(cur.left)
                    layer_node.append(cur.right)
            
            node_lst.extend(layer_node)
                
        return True