用python重写下线段树模板，线段树的记忆点在于脑子里要有线段树，特点是自顶向下构建
class segment_tree:
def __init__(self, nums):
n = len(nums)
self.nums = nums
self.tree = [0] * 4 * n
self.tag = [0] * 4 * n
def update(self, root):
self.tree[root] = self.tree[root * 2] + self.tree[root * 2