求二叉树每一层的最大值，按层BFS
1 #515
2 #Runtime: 26 ms (Beats 68.6%)
3 #Memory: 17.7 MB (Beats 71.73%)
4
5 # Definition for a binary tree node.
6 # class TreeNode(object):
7 # def __init__(self, val=0, left=None, right=None):
8 # self.val = val
9 # self.left = left
10 # self.right = right
11 class Solution(object):
12 def largestValues(self, root):
13 """
14 :type root: TreeNode
15 :rtype: List[int]
16 """
17 if not root:
18 return []
19 q = deque([root])
20 ans = []
21 while q:
22 sz = len(q)
23 t = float('-inf')
24 while sz:
25 sz -= 1
26 node = q.popleft()
27 t = max(t, node.val)
28 if node.left:
29 q.append(node.left)
30 if node.right:
31 q.append(node.right)
32 ans.append(t)
33 return ans