[LeetCode] 1448. Count Good Nodes in Binary Tree

 Problem : https://leetcode.com/problems/count-good-nodes-in-binary-tree/

Use DFS to find number of nodes where value not less than the current local maximum value.

# 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:
        
        result = 0
        stack = [(root, -10**4)]
        
        while stack:
            node, mx = stack.pop()
            
            if mx <= node.val:
                result += 1
                mx = node.val
            
            if node.right:
                stack.append((node.right, mx))
            
            if node.left:
                stack.append((node.left, mx))
        
        return result