题目描述：给定一个二叉树的 根节点 root，想象自己站在它的右侧，按照从顶部到底部的顺序，返回从右侧所能看到的节点值。

解题思路：bfs，dfs

解法一：bfs

class Solution {
    public List<Integer> rightSideView(TreeNode root) {
        List<Integer> ans = new LinkedList<>();
        if (root == null) {
            return ans;
        }
        Deque<TreeNode> deque = new LinkedList<>();
        deque.offer(root);
        while (!deque.isEmpty()) {
            int size = deque.size();
            for (int i = 0; i < size; i++) {
                TreeNode tmp = deque.poll();
                if (i + 1 == size) {
                    ans.add(tmp.val);
                }
                if (tmp.left != null) {
                    deque.offer(tmp.left);
                }
                if (tmp.right != null) {
                    deque.offer(tmp.right);
                }
            }
        }
        return ans;
    }
}

解法二：dfs

class Solution {
    // DFS
    List<Integer> res = new LinkedList<>();
    public List<Integer> rightSideView(TreeNode root) {
        dfs(root, 0);
        return res;
    }
    private void dfs(TreeNode root, int depth) {
        if (root == null) {
            return;
        }
        if (depth == res.size()) {
            res.add(root.val);
        }
        depth++;
        dfs(root.right, depth);
        dfs(root.left, depth);
    }
}