1973. Count Nodes Equal to Sum of Descendants

# 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 equalToDescendants(self, root: Optional[TreeNode]) -> int:
        self.count=0
        
        def dfs(node):
            if not node:
                return 0
            add=dfs(node.left)+dfs(node.right)
            if node.val==add:
                self.count+=1
            
            return add+node.val
        
        dfs(root)
        return self.count