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Help Needed
By ___Abc123___, history, 2 hours ago, In English

Hi, Here is the Question You are given an array A of length N and q queries where each query is one of the following types:

1 i val: Update the value at ith index to val (arr[i] = val)

   2 L R: find the sum of all the subarray of arr[L....R]

Determine the value of query 2 Ex: N = 5

q 2;

arr = [2, 1, 4, 3, 1]

query = [(1, 2, 2), (2, 1, 3)]

Output = 26

How to approach this question

I have to find sum of every subarray of arr[L...R] ans will be arr[L] + arr[L...(L + 1)] + arr[L...(L +2)] +.....arr[L....R] + arr[L + 1] + (arr(L + 1)..(L + 2)] +.....(arr[(L + 1)...R])......

constraint 1 <= N, q <= 100000

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2 hours ago, # |
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Auto comment: topic has been updated by ___Abc123___ (previous revision, new revision, compare).

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2 hours ago, # |
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is this not just a standard seg/fenwick tree problem try learning one of these datastructures, then it will be a template problem

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2 hours ago, # |
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Check https://leetcode.com/problems/range-sum-query-mutable/description/.

People there have given great explanations for this problem already.

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2 hours ago, # |
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I have to find sum of every subarray of arr[L...R] ans will be arr[L] + arr[L...(L + 1)] + arr[L...(L +2)] +.....arr[L....R] + arr[L + 1] + (arr(L + 1)..(L + 2)] +.....(arr[(L + 1)...R])......

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    2 hours ago, # ^ |
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    interestingly i have actually seen this problem before

    try thinking about it as how many times each element in the range contributes to the final sum

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      115 minutes ago, # ^ |
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      Yes I have done that final expression will be (r — l + 1)arr[l] + 2*(r — l)arr[l + 1] + 3*(r — l — 1)*arr[l + 2]+.....+(r — l + 1)arr[r].

      still cant figure out what to do from here

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        66 minutes ago, # ^ |
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        Another way to look at it would be to only check the contribution of each individual element in $$$[L,R]$$$. Here the $$$ith$$$ elements contribution would be $$$arr[i]*(r-i+1)*(i-l+1)$$$ as these many subarrays in $$$[L,R]$$$ contain it. Now you can see that you only need the sum of $$$arr[i]*i, arr[i]*i*i, arr[i]$$$ to find the contribution of all elements which you can find using a segment tree.

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2 hours ago, # |
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Auto comment: topic has been updated by ___Abc123___ (previous revision, new revision, compare).

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72 minutes ago, # |
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Click Here

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56 minutes ago, # |
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Try learning about Fenwick Tree(Binary Indexed Tree) or Segment trees! :)

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