rangeSumBST-func-simple.py

import operator
from functools import partial, reduce

def flatten(node):
    if not node:
        return []
    return flatten(node.left) + [node.val] + flatten(node.right)

def rangeSumBST(root, L, R):
    return reduce(operator.add,
                  filter(partial(operator.ge, R),
                         filter(partial(operator.le, L), flatten(root))))