Given a binary tree, return thebottom-up level ordertraversal 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]
]

tag: binary tree, BFS

/**
 * 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>> ans = new ArrayList<>();
        if (root == null) return ans;
        Queue<TreeNode> q = new LinkedList<>();

        q.offer(root);

        while (!q.isEmpty()){
            int size = q.size();
            List<Integer> list = new ArrayList<>();

            for (int i = 0; i < size; i++){
                TreeNode cur = q.poll();
                list.add(cur.val);

                if (cur.left != null){
                    q.offer(cur.left);
                }
                if (cur.right != null){
                    q.offer(cur.right);
                }
            }
            ans.add(0, list);
        }


        return ans;
    }
}