Question Link:

https://leetcode.com/problems/binary-tree-level-order-traversal-ii/

Use BFS from the tree root to traverse the tree level by level. The python code is as follows.

[-] Python code accepted by LeetCode OJ 
# Definition for a  binary tree node
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    # @param root, a tree node
    # @return a list of lists of integers
    def levelOrderBottom(self, root):
        """
        BFS from root, and record the values level by level
        """
        # List of levels
        res = []
        # Special case: empty tree
        if not root:
            return res
        level = [root]
        while level:
            temp = []
            res.insert(0, [n.val for n in level])
            for node in level:
                if node.left:
                    temp.append(node.left)
                if node.right:
                    temp.append(node.right)
            level = temp
        # Return
        return res