python binary index tree

gistfile1.py

from __future__ import division

class BinaryIndexTree:
    def __init__(self, n):
        self.sz = n
        self.vals = [0] * (n + 1)

    def update(self, idx, delta):
        "add c to the value at index idx"
        while idx <= self.sz:
            self.vals[idx] += delta
            idx += idx & (-idx)

    def accumulate(self, idx):
        "get sum from the start to the index of idx"
        ret = 0
        while idx > 0:
            ret += self.vals[idx]
            idx -= idx & (-idx)
        return ret

    def range_sum(self, start, end):
        "Calculate a[start], a[start+1], ... a[end]"
        return self.accumulate(end) - self.accumulate(start - 1)



def test():
    bit = BinaryIndexTree(10)
    assert bit.range_sum(1, 5) == 0
    bit.update(1, 3)
    bit.update(4, 6)
    assert bit.range_sum(1, 4) == 9
    bit.update(3, 3)
    assert bit.range_sum(1, 4) == 12
    assert bit.range_sum(3, 4) == 9
    bit.update(3, -2)
    assert bit.range_sum(3, 3) == 1
    assert bit.range_sum(1, 4) == 10
    return 'test pass'

if __name__ == '__main__':
    print test()

You maked a mistake. line 11 should be while idx<=self.sz, your index start from 0

@Haozun Thanks for pointing out the problem. Fix the mistake. But I think that's because my index start from 1 not 0.