LeetCode-right view of binary tree

Q: Given a binary tree, imagine yourself standing on the right side of it, returning the node values ​​that can be seen from the right side, in order from top to bottom.

A:
Hierarchical traversal, take the last one of each layer.

public List<Integer> rightSideView(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null)
            return res;
        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);
        int size = 1;
        while (!q.isEmpty()) {
            while (size != 0) {
                if (size == 1) {
                    res.add(q.peek().val);
                }
                TreeNode curr = q.poll();
                if (curr.left != null) {
                    q.add(curr.left);
                }
                if (curr.right != null) {
                    q.add(curr.right);
                }
                size--;
            }
            size = q.size();
        }
        return res;

    }