# 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 diameterOfBinaryTree(self, root: Optional[TreeNode]) -> int:
        result = [0]
        
        def dfs(node):
            # Leaf node height will be 1
            # Empty node height will be -1
            
            if node is None: return -1
            
            lefth = dfs(node.left)
            righth = dfs(node.right)
            
            # Diameter calc:
            #
            # Cause for current node will be pointing to both
            # left and right nodes and we need to consider
            # them (+2)
            #
            # e.g. consider this is leaf node, lefth and righth = -1
            # so, current node's diameter should be 0 = 2 + -1 + -1
            result[0] = max(result[0], 2 + lefth + righth)
            
            # return max height including the node itself
            return 1 + max(lefth, righth)
        
        dfs(root)
        
        return result[0]