[LeetCode 中等 二叉树]199. 二叉树的右视图

题目描述

给定一棵二叉树，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。

示例:
输入: [1,2,3,null,5,null,4]
输出: [1, 3, 4]
解释:

   1            <---
 /   \
2     3         <---
 \     \
  5     4       <---

前序遍历将最后一个加入答案集合

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> rightSideView(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if(root==null) return res;

        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);

        while(!queue.isEmpty()){
            int count = queue.size();
            while(count>0){
                TreeNode cur = queue.remove();
                if(count==1) res.add(cur.val);
                if(cur.left != null) queue.offer(cur.left);
                if(cur.right != null) queue.offer(cur.right);
                count--;
            }
        }
        return res;
    }
}

深度优先搜索

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    List<Integer> res = new ArrayList<>();
    
    public List<Integer> rightSideView(TreeNode root) {
        recursion(root,0);
        return res;
    }

    public void recursion(TreeNode root,int depth){
        if(root == null) return ;
        if(res.size() == depth) res.add(root.val);
        depth++;
        recursion(root.right,depth);
        recursion(root.left,depth);
    }
}