LazySegmentTree Issue

[akshitm169]

Hi, I was trying LazySegmentTree on D. Addition and Sum but getting different output.

My Code

from sys import stdin, stdout
input = stdin.readline

class LazySegmentTree:
    def __init__(self, data, default=0, func= lambda a,b: a+b):
        """initialize the lazy segment tree with data"""
        self._default = default
        self._func = func

        self._len = len(data)
        self._size = _size = 1 << (self._len - 1).bit_length()
        self._lazy = [0] * (2 * _size)

        self.data = [default] * (2 * _size)
        self.data[_size:_size + self._len] = data
        for i in reversed(range(_size)):
            self.data[i] = func(self.data[i + i], self.data[i + i + 1])

    def __len__(self):
        return self._len

    def _push(self, idx):
        """push query on idx to its children"""
        # Let the children know of the queries
        q, self._lazy[idx] = self._lazy[idx], 0

        self._lazy[2 * idx] += q
        self._lazy[2 * idx + 1] += q
        self.data[2 * idx] += q
        self.data[2 * idx + 1] += q

    def _update(self, idx):
        """updates the node idx to know of all queries applied to it via its ancestors"""
        for i in reversed(range(1, idx.bit_length())):
            self._push(idx >> i)

    def _build(self, idx):
        """make the changes to idx be known to its ancestors"""
        idx >>= 1
        while idx:
            self.data[idx] = self._func(self.data[2 * idx], self.data[2 * idx + 1]) + self._lazy[idx]
            idx >>= 1

    def add(self, start, stop, value):
        """lazily add value to [start, stop)"""
        start = start_copy = start + self._size
        stop = stop_copy = stop + self._size
        while start < stop:
            if start & 1:
                self._lazy[start] += value
                self.data[start] += value
                start += 1
            if stop & 1:
                stop -= 1
                self._lazy[stop] += value
                self.data[stop] += value
            start >>= 1
            stop >>= 1

        # Tell all nodes above of the updated area of the updates
        self._build(start_copy)
        self._build(stop_copy - 1)

    def query(self, start, stop, default=0):
        """func of data[start, stop)"""
        start += self._size
        stop += self._size

        # Apply all the lazily stored queries
        self._update(start)
        self._update(stop - 1)

        res = default
        while start < stop:
            if start & 1:
                res = self._func(res, self.data[start])
                start += 1
            if stop & 1:
                stop -= 1
                res = self._func(res, self.data[stop])
            start >>= 1
            stop >>= 1
        return res

    def __repr__(self):
        return "LazySegmentTree({0})".format(self.data)

n,m=map(int,input().split())
arr=[0 for x in range(n)]
ST=LazySegmentTree(arr)
for _ in range(m):
    ops=[int(x) for x in input().split()]
    if ops[0]==1:
        ST.add(ops[1],ops[2],ops[3])
    else:
        print(ST.query(ops[1],ops[2]))

Input:

5 6
1 0 3 3
2 1 2
1 1 4 4
2 1 3
2 1 4
2 3 5

Expected Output:

3
14
18
4

Output of above code:

3
14
14
4

[bjorn-martinsson]

That lazy segment tree code is written to be used with max, not addition.

To make it be able to handle addition you need to make some modifications. Change

        self.data[2 * idx] += q
        self.data[2 * idx + 1] += q
...
        self.data[start] += value
...
       self.data[stop] += value

to

        self.data[2 * idx] += q * interval_length[2 * idx]
        self.data[2 * idx + 1] += q * interval_length[2 * idx + 1]
...
        self.data[start] += value * interval_length[start]
...
       self.data[stop] += value * interval_length[stop]

where interval_length[i] is the length of the interval corresponding to segment tree node i.

[akshitm169]

Thank you very much!!