Binary Tree Zigzag Level Order Traversal

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

For example: Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its zigzag level order traversal as:

[
  [3],
  [20,9],
  [15,7]
]

class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> ans = new ArrayList<>();
        if (root == null)
            return ans;
        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);
        int values = 1;
        boolean rightToLeft = false;
        while (q.size() != 0) {
            ArrayList<Integer> temp = new ArrayList<>();
            int tempCounter = values;
            values = 0;
            while (tempCounter-- > 0) {
                TreeNode top = q.poll();
                if (rightToLeft)
                    temp.add(0, top.val);
                else
                    temp.add(top.val);
                if (top.left != null) {
                    q.add(top.left);
                    values++;
                }
                if (top.right != null) {
                    q.add(top.right);
                    values++;
                }
            }
            ans.add(temp);
            rightToLeft = !rightToLeft;
        }
        return ans;
    }
}