Diameter of Binary Tree

def diameterOfBinaryTree(root):
    res = 0
    
    def dfs(node):
        nonlocal res
        if not node:
            return 0
            
        left = dfs(node.left)
        right = dfs(node.right)
        
        res = max(res, left + right)
        return 1 + max(left, right)
        
    dfs(root)
    return res