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Understanding Prefix Sum with a Practical Example (Pocket Money Tracker)

Prefix Sum is a simple but powerful technique used in many array problems instead of recalculating sums repeatedly, we store running totals to answer queries instantly.Let’s understand this using a small real-life example. The Scenario: Pocket Money Tracker Imagine you are tracking your daily expenses for a week.You store them in an array: expenses = [10, 20, 5, 15, 30, 10, 50] Each number represents the amount spent on that day. Day Expense 0 10 1 20 2 5 3 15 4 30 5 10 6 50 Now your friend asks questions like: How much did you spend from Day 2 to Day 4? What was the total from Day 0 to Day 5? If you calculate this every time using a loop, it becomes inefficient. This is where Prefix Sum helps. Step 1: Build the Prefix Sum Array Prefix sum stores the running total of expenses. Formula: prefix[i] = prefix[i-1] + expenses[i] Result: Day Prefix Sum 0 10 1 30 2 35 3 50 4 80 5 90 6 140 So the prefix a
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