Symmetric Tree

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public boolean isSymmetric(TreeNode root) {
        if(root == null) return true;
        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);
        while(!q.isEmpty()){
            int s = q.size();
            List<Integer> lvl = new ArrayList<>();
            for(int i = 0; i<s; i++){
                TreeNode cur = q.poll();
                if(cur.left != null){
                    lvl.add(cur.left.val);
                    q.add(cur.left);
                }
                else{
                    lvl.add(-1234231);
                }
                if(cur.right != null){
                    lvl.add(cur.right.val);
                    q.add(cur.right);
                }
                else{
                    lvl.add(-1234231);
                }
            }
            //System.out.println(lvl);
            if(!isPal(lvl)){
                return false;
            }
        }
        return true;
    }
    public boolean isPal(List<Integer> lvl){
        for(int i = 0; i<lvl.size()/2; i++){
            if(!lvl.get(i).equals(lvl.get(lvl.size()-i-1))){
                //System.out.println(lvl.get(i) + " " + lvl.get(lvl.size()-i-1));
                return false;
            }
        }
        return true;
    }
}