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).

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> res = new LinkedList<List<Integer>>();
        if(root == null)
            return res;

        LinkedList<TreeNode> curr = new LinkedList<>();
        LinkedList<TreeNode> next = new LinkedList<>();
        List<Integer> list = new ArrayList<Integer>();

        curr.offer(root);

        while(curr.size() > 0) {
            TreeNode temp = curr.poll();
            list.add(temp.val);

            if(temp.left != null)
                next.add(temp.left);
            if(temp.right != null)
                next.add(temp.right);

            if(curr.size() == 0) {
                curr = next;
                next = new LinkedList<>();
                res.add(0, list);
                list = new ArrayList<>();
            }
        }
        return res;
    }
}