Leetcide problem doubt

Here in the solution why we are addingt the start location and subtracting the end location

What we are doing basically is keeping track of the number of passengers at any given time as a prefix sum. So if a trip i starts at t1, we add the passengers at time t1 for the trip. Now they stay in the car till time t2, so we subtract them at t2. Now at any point just simply check if sum is greater than capacity or not.

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