Leetcode Binary Tree Level Order Traversal II

public class Solution {
public List<List<Integer>> levelOrderBottom(TreeNode root) {
List<List<Integer>> ret = new ArrayList<>();
if (root == null) return ret;

Queue<TreeNode> queue = new LinkedList<>();
Stack<ArrayList<Integer>> stack = new Stack<>();
queue.offer(root);

// do the level per level BFS
while (!queue.isEmpty()) {
ArrayList<Integer> level = new ArrayList<>();
int size = queue.size();
for (int i=0; i<size; i++) {
TreeNode node = queue.poll();
level.add(node.val);
if (node.left != null)
queue.offer(node.left);
if (node.right != null)
queue.offer(node.right);
}
stack.push(level);
}

// pop levels in the stack
while (!stack.isEmpty()) {
ret.add(stack.pop());
}

return ret;
}
}