199.二叉树的右视图

思路：

使用根->右->左方法进行遍历节点，同时记录层数，将当前层数与记录的层数进行比较，如果当前层数大于记录的层数，添加该元素，若当前层数小于记录的层数，说明该层已经有元素被记录，就跳过，这样保证每层第一个被记录的一定是最右边的元素

/**
 * 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 {
    int maxHigh = 0;
    List<Integer> res = new ArrayList<Integer>();
    public List<Integer> rightSideView(TreeNode root) {
        dfs(root,1);
        return res;
    }

    public void dfs(TreeNode root,int high){
        if(root == null){
            return ;
        }
        if(maxHigh < high){
            res.add(root.val);
            maxHigh = high;
        }
        dfs(root.right,high + 1);
        dfs(root.left,high + 1);
    }
    
}