leetcode算法： Find Largest Value in Each Tree Row

'''
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

这道题描述的是：
　　给我们一颗二叉树的根节点，返回每一层最大值的数组

思想：
　　广度优先遍历，把每一层最大值放入结果列表

python实现：

 class Solution(object):
     def largestValues(self, root):
         """
         :type root: TreeNode
         :rtype: List[int]
         """
         if not root:
             return []
         levels = [[root]]
         values = [[root.val]]
         for level in levels:
             next_level = []
             value = []
             for node in level:
                 if node.left is not None:
                     next_level.append(node.left)
                     value.append(node.left.val)
                 if node.right is not None:
                     next_level.append(node.right)
                     value.append(node.right.val)
             if next_level:
                 levels.append(next_level)
                 values.append(value)
         res = []
         for v in values:
             res.append(max(v))
         return res

精简一下代码：

 class Solution(object):
     def largestValues(self, root):
         """
         :type root: TreeNode
         :rtype: List[int]
         """
         values = []
         level = [root]
         while any(level):
             values.append( max( [i.val for i in level] ) )
             level = [child for node in level for child in [node.left, node.right ] if child is not None]
         return values