‘‘‘
You need to find the largest value in each row of a binary tree.

Example:
Input:

          1
         / \
        3   2
       / \   \
      5   3   9

Output: [1, 3, 9]


‘‘‘

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

#         self.right = None


這道題描述的是：
　　給我們一顆二叉樹的根節點，返回每一層最大值的數組


思想：
　　廣度優先遍歷，把每一層最大值放入結果列表

python實現：

 1 class Solution(object):
 2     def largestValues(self, root):
 3         """
 4         :type root: TreeNode
 5         :rtype: List[int]
 6         """
 7         if not root:
 8             return []
 
 9         levels = [[root]]
10         values = [[root.val]]
11         for level in levels:
12             next_level = []
13             value = []
14             for node in level:
15                 if node.left is not None:
16                     next_level.append(node.left)
17                     value.append(node.left.val)
 
18                 if node.right is not None:
19                     next_level.append(node.right)
20                     value.append(node.right.val)
21             if next_level:
22                 levels.append(next_level)
23                 values.append(value)
24         res = []
25         for v in values:
26             res.append(max(v))
27         return res

精簡一下代碼：

 1 class Solution(object):
 2     def largestValues(self, root):
 3         """
 4         :type root: TreeNode
 5         :rtype: List[int]
 6         """
 7         values = []
 8         level = [root]
 9         while any(level):
10             values.append( max( [i.val for i in level] ) )
11             level = [child for node in level for child in [node.left, node.right ] if child is not None]
12         return values

leetcode算法： Find Largest Value in Each Tree Row