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DSA Lecture 7: Mastering Prefix Sum, The Foundation of Fast Queries
Have you ever kept track of your daily expenses in a notebook or app?
Say you spend $100 on Day 1, $200 on Day 2, $50 on Day 3… and by Day 10, you’re wondering:
“How much did I spend between Day 3 and Day 7?”
Now imagine you do this calculation again and again.
You either keep summing the values every time (slow), or you build a “cumulative total” once and answer any future query instantly.
That’s exactly where Prefix Sum (also called Cumulative Sum) comes in.
From spreadsheets calculating totals, to analytics tools generating charts, and even in DSA interviews, this powerful technique helps us answer range sum queries fast.(If you don’t have premium access, read here)
Let’s learn how to think in Prefix, how to code it smartly, and how this tiny trick saves time big in both interviews and real life.
What is Prefix Sum (Cumulative Sum) & its Working?
Imagine This:
You have an array of numbers representing daily income for a week:arr = [100, 200, 150, 300, 250, 100, 200]
