107. Binary Tree Level Order Traversal II

 1 class Solution {
 2     public List<List<Integer>> levelOrderBottom(TreeNode root) {
 3         Queue<TreeNode> queue = new LinkedList<>();
 4         List<List<Integer>> res = new ArrayList<>();
 5         if(root == null) return res;
 6         queue.offer(root);
 7         int size = queue.size();
 8         List<Integer> list = new ArrayList<>();
 9         while(!queue.isEmpty()) {
10             for(int i = 0; i < size; i++) {
11                 TreeNode node = queue.poll();
12                 list.add(node.val);
13                 if(node.left != null) {
14                     queue.offer(node.left);
15                 }
16                 if(node.right != null) {
17                     queue.offer(node.right);
18                 }
19             }
20             size = queue.size();
21             res.add(0, list);
22             list = new ArrayList<>();
23         }
24         return res;
25     }
26 }