leetcode 199 binary tree right side view

Given a binary tree, imagine yourself standing on the right side of it, return the values of the nodes you can see ordered from top to bottom.

Example:

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Input: [1,2,3,null,5,null,4]
Output: [1, 3, 4]
Explanation:

1 <---
/
2 3 <---

5 4 <---

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* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class {
public List<Integer> rightSideView(TreeNode root) {
List<Integer> ans = new ArrayList<>();
helper(ans, root, 0);
return ans;
}
public void helper(List<Integer> ans, TreeNode root, int level) {
if (root == null)
return ;
if (ans.size() == level) {
ans.add(root.val);
}
if (root.right != null)
helper(ans, root.right, level+1);
if (root.left != null)
helper(ans, root.left, level+1);
}
}