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);}}