Problem Description

Given a binary tree root, a node X in the tree is named good if in the path from root to X there are no nodes with a value greater than X.

Return the number of good nodes in the binary tree.

Examples

Example 1:

Input: root = [3,1,4,3,null,1,5]
Output: 4
Explanation: Nodes in blue are good.
Root Node (3) is always a good node.
Node 4 -> (3,4) is the maximum value in the path starting from the root.
Node 5 -> (3,4,5) is the maximum value in the path
Node 3 -> (3,1,3) is the maximum value in the path.

Example 2:

Input: root = [3,3,null,4,2]
Output: 3
Explanation: Node 2 -> (3, 3, 2) is not good, because "3" is higher than it.

Example 3:

Input: root = [1]
Output: 1
Explanation: Root is considered asgood.

Constraints:

• The number of nodes in the binary tree is in the range $[1, 10^5]$.
• Each node's value is between $[-10^4, 10^4]$.

解析

題目給一個二元樹的根結點 root

定義一個結點 X 是 good 代表從 root 到 X 的值都不大於 X.Val

要累計這個二元樹是 good 的結點個數

關鍵點在於透過 DFS 還有累計當下最大的值來做符合條件的檢驗