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.

For example:
Given the following binary tree,
   1            <---
 /   
2     3         <---
      
  5     4       <---
You should return [1, 3, 4].

Credits:
Special thanks to @amrsaqr for adding this problem and creating all test cases.

算法: 根右左, 尾递归

容器: list 尾递归

corner case: 右没有怎么办, 加一个输入值表示深度,  list.size() 也表示深度

The core idea of this algorithm:

1.Each depth of the tree only select one node.

2.View depth is current size of result list.

 public List<Integer> rightSideView(TreeNode root) {
        List<Integer> ans = new ArrayList<Integer>();
        if (root == null) {
            return ans;
        }
        helper(root, ans, 0);
        return ans;
       
    }
    private void helper(TreeNode root, List<Integer> ans, int size) {
        if (root == null) {
            return;
        }
        if (size == ans.size()) {
            ans.add(root.val);
        }
        helper(root.right, ans, size + 1);
        helper(root.left, ans, size + 1);
    }
   

是递的时候操作题意还是归的时候操作? --->尾递归? 输入值? 还是分治? , 辅助全局变量?