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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.
思路:BFS层序遍历
# 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