diameterOfBinaryTree.py

def diameterOfBinaryTree(self, root):
    self.res = 0
    def helper(root):
        if not root: return 0
        left, right = helper(root.left), helper(root.right)
        self.res = max(self.res, left + right)
        return 1 + max(left, right)
    helper(root)
    return self.res