107.binary-tree-level-order-traversal-ii.python3.py

#
# [107] Binary Tree Level Order Traversal II
#
# https://leetcode.com/problems/binary-tree-level-order-traversal-ii/description/
#
# algorithms
# Easy (43.61%)
# Total Accepted:    187K
# Total Submissions: 425.4K
# Testcase Example:  '[3,9,20,null,null,15,7]'
#
# Given a binary tree, return the bottom-up level order traversal of its nodes'
# values. (ie, from left to right, level by level from leaf to root).
# 
# 
# For example:
# Given binary tree [3,9,20,null,null,15,7],
# 
# ⁠   3
# ⁠  / \
# ⁠ 9  20
# ⁠   /  \
# ⁠  15   7
# 
# 
# 
# return its bottom-up level order traversal as:
# 
# [
# ⁠ [15,7],
# ⁠ [9,20],
# ⁠ [3]
# ]
# 
# 
#
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def levelOrderBottom(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        res = []
        if root is None:
            return res 
        
        queue = []
        queue.append(root)
        while queue:
            cur_level_width = len(queue)
            cur_levle_item = []
            while cur_level_width:
                temp = queue.pop(0)
                cur_levle_item.append(temp.val)
                if temp.left:
                    queue.append(temp.left)
                if temp.right:
                    queue.append(temp.right)
                cur_level_width -= 1
            res.append(cur_levle_item)
        return res[::-1]