# 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:
        ans = 0

        def dfs(node: TreeNode) -> int:
            nonlocal ans

            left = 0
            if node.left:
                left = 1 + dfs(node.left)
            right = 0
            if node.right:
                right = 1 + dfs(node.right)

            ans = max(ans, left + right)

            return max(left, right)

        if root:
            dfs(root)

        return ans