先递归右子树, 再递归左子树, 记录层高, 第一次到达该层的时候, 记录答案 @param root @return

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode() {}
    TreeNode(int val) { this.val = val; }
    TreeNode(int val, TreeNode left, TreeNode right) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}
// @lc code=start
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    /**
     * 先递归右子树, 再递归左子树, 记录层高,
     * 第一次到达该层的时候, 记录答案
     * @param root
     * @return
     */
    public List<Integer> rightSideView(TreeNode root) {
        List<Integer> ans = new ArrayList<>();
        dfs(ans, root, 0);
        return ans;
    }

    private void dfs(List<Integer> ans, TreeNode root, int level) {
        if (root == null) return ;
        if (level == ans.size()) {
            ans.add(root.val);
        }
        dfs(ans, root.right, level+1);
        dfs(ans, root.left, level+1);
    }
}