Leetcode 199.二叉树的右视图（Binary Tree Right Side View）

Leetcode 199.二叉树的右视图

1 题目描述（Leetcode题目链接）

  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。

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

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

2 题解

  层次遍历保留最右边的值。

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def rightSideView(self, root: TreeNode) -> List[int]:
        if not root:
            return []
        queue = collections.deque([root])
        res = []
        while queue:
            num = len(queue)
            while num:
                node = queue.popleft()
                if node.left:
                    queue.append(node.left)
                if node.right:
                    queue.append(node.right)
                num -= 1
                if num == 0:
                    res.append(node.val)
        return res

  深度优先搜索，存字典。

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def rightSideView(self, root: TreeNode) -> List[int]:
        res = {}
        def dfs(root, depth):
            if root:
                res[depth] = root.val
                dfs(root.left, depth + 1)
                dfs(root.right, depth + 1)
        
        dfs(root, 0)
        return list(res.values())