fenwick树

fenwick = [0]*(max_len+1)
def (x):
    return x&(-x)
def add(pos, value):
    while pos <= max_len:
        fenwick[pos] += value
        pos += lowbit(pos)
def sum(pos):
    res = 0
    while pos != 0:
        res += fenwick[pos]
        pos -= lowbit(pos)
    return res