107. Binary Tree Level Order Traversal II

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]
]

题目含义：这道题的要求是从下往上分层遍历二叉树。

    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> result = new LinkedList<List<Integer>>();
        Queue<TreeNode> queue = new LinkedList<>();
        if (root == null) return result;
        queue.add(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            List<Integer> values = new ArrayList<>();
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                if (node.left != null) queue.offer(node.left);
                if (node.right != null) queue.offer(node.right);
                values.add(node.val);
            }
            result.add(0, values);
        }
        return result;     
    }