107. 二叉树的层次遍历 II

class Solution(object):
    def levelOrderBottom(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        if not root:
            return []
        level_queue = []
        ans = []
        level_queue.append(root)
        while level_queue:
            num = len(level_queue)
            temp = []
            while num > 0:
                cur = level_queue.pop(0)
                temp.append(cur.val)
                if cur.left:
                    level_queue.append(cur.left)
                if cur.right:
                    level_queue.append(cur.right)
                num -= 1
            ans.append(temp)
        return ans[::-1]