107. Binary Tree Level Order Traversal II Java Solutions

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        Stack<List<Integer>> stack = new Stack<List<Integer>>();
        Queue<TreeNode> q = new LinkedList<TreeNode>();
        if(root == null) return res;
        q.add(root);
        while(!q.isEmpty()){
            List<Integer> tmp = new ArrayList<Integer>();
            int size = q.size();
            for(int i = 0; i< size; i++){
                TreeNode node = q.poll();
                tmp.add(node.val);
                if(node.left != null) q.add(node.left);
                if(node.right != null) q.add(node.right);
            }
            stack.push(tmp);
            
        }
        while(!stack.isEmpty()){
            res.add(stack.pop());
        }
        return res;
    }
}