1448. Count Good Nodes in Binary Tree

Medium

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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.

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 as good.

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].

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def goodNodes(self, root: TreeNode) -> int:
        return self.dfs(root, root.val)
    
    def dfs(self, root, val):
        if root is None:
            return 0
        result = 1 if root.val >= val else 0
        if root.left:
            result += self.dfs(root.left, max(root.val, val))
        if root.right:
            result += self.dfs(root.right, max(root.val, val))
        return result