Prefix Sum Array and Range Sum Queries 

Prefix Sum Arrays or simply Prefix Sums are used to perform fast range sum queries on a given array. The total time complexity of the algorithm is O(N + Q) where N is the number of elements in the given array and Q is the number of range sum queries. Each of the Q queries are answered in O(1) time.
This topic often appears during technical interviews at top tech companies (like Google, Facebook, Amazon, etc). You can prepare for those interviews by practicing and solving challenging problems by following the links below.
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Chapters:
0:00 Prefix Sum and Range Sum Queries Problem Statement
0:18 Example of Video Performance
0:58 Naive Approach
1:29 Reformulation of the Problem
2:02 Naive Prefix Sum Array Calculation
3:41 Prefix Sum Array Calculation
4:36 Range Sum Queries
5:20 Problem with L being 0
5:42 The Final Algorithm
6:51 Time and Memory Complexity
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