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],

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。

 1     public List<List<Integer>> levelOrderBottom(TreeNode root) {
 2         List<List<Integer>> result = new LinkedList<List<Integer>>();
 3         Queue<TreeNode> queue = new LinkedList<>();
 4         if (root == null) return result;
 5         queue.add(root);
 6         while (!queue.isEmpty()) {
 7             int size = queue.size();
 8             List<Integer> values = new ArrayList<>();
 9             for (int i = 0; i < size; i++) {
10                 TreeNode node = queue.poll();
11                 if (node.left != null) queue.offer(node.left);
12                 if (node.right != null) queue.offer(node.right);
13                 values.add(node.val);
14             }
15             result.add(0, values);
16         }
17         return result;     
18     }