[leetcode] 958. Check Completeness of a Binary Tree @ python

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原题

https://leetcode.com/problems/check-completeness-of-a-binary-tree/

解法

BFS. 按层将节点(包括空节点)放入队列, 依次从左边取出节点, 如果已发现一个空节点, 并且空节点后面还有节点, 返回False.

Time: O(n), n为节点数
Space: O(n)

代码

# 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
        """
        # base case
        if not root: return True
        q = [root]
        missing = False
        while q:
            size = len(q)
            while size > 0:
                node = q.pop(0)
                size -= 1
                if node is None:
                    missing = True
                else:
                    # if there exists a node after a missing node
                    if missing:
                        return False
                    q.append(node.left)
                    q.append(node.right)
                    
        return True