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:

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

   1            <---
 /   \
2     3         <---
 \     \
  5     4       <---

preorder traverse; right -> left

class Solution {
  int deepest = 0;
   public List<Integer> rightSideView(TreeNode root) {
       ArrayList<Integer> result = new ArrayList<>();
       traverse(root, result, 1);
       return result;
   }

  public void traverse(TreeNode root, ArrayList<Integer> result, int level) {
      if(root == null) return;
      if(level == deepest + 1) {
          result.add(root.val);
          deepest = level;
      }
      traverse(root.right, result, level+1);
      traverse(root.left, result, level+1);
  }
}