binary-tree-level-order-traversal-ii.py

# Given the root of a binary tree, return the bottom-up level order traversal of its nodes' values. (i.e., from left to right, level by level from leaf to root).
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# Example 1:
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# Input: root = [3,9,20,null,null,15,7]
# Output: [[15,7],[9,20],[3]]
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# Example 2:
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# Input: root = [1]
# Output: [[1]]
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# Example 3:
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# Input: root = []
# Output: []
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# Constraints:
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# 	The number of nodes in the tree is in the range [0, 2000].
# 	-1000 <= Node.val <= 1000
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# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def levelOrderBottom(self, root: TreeNode) -> List[List[int]]:
        if root is None:
            return []
        queue = [root]
        res = []
        while queue:
            nextLevel = []
            for node in queue:
                if node.left:
                    nextLevel.append(node.left)
                if node.right:
                    nextLevel.append(node.right)
            res.append([node.val for node in queue])
            queue = nextLevel
        return res[::-1]