Explanation of the code:

Each operation has to be applied to a range of values, i.e., to the values in array having indices starting from p to q. Here, in the first loop, the value to be added (sum variable) is added to the starting location p in the array, and the sum is subtracted from the ending location + 1, i.e., q + 1. In the second loop, the value added to the starting location gets added to successive locations, i.e., it gets added to p, p+1, p+2, .... until q, because each value that was added in the starting location using first loop, will be added to the variable x and therefore in effect gets added to subequent locations. But once the range ends, i.e., once you reach q+1, the sum that was added at starting location will be subtracted. This will subtract the sum value from the variable x in which the sum value was added at starting location p, meaning the addition of sum will only affect locations p to q.

Example:

If n=5 and operations are:-

1 2 100
2 5 100
3 4 100

Initial state of array (starting from index 1 until 5):-

0 0 0 0 0

Applying first operation

• Add 100 (sum) at index 1 (p)
• Subtract 100 (sum) from index 3 (q+1)

Result

100 0 -100 0 0

Applying second operation - Add 100 (sum) at index 2 (p) - Do NOT subtract 100 (sum) from index 6 (q+1) since 6 is not lesser or equal to N (refer to code)

Result

100 100 -100 0 0

Applying third operation - Add 100 (sum) at index 3 (p) - Subtract 100 (sum) from index 5 (q+1)

Result

100 100 0 0 -100

If you start adding values from this array to a variable x, the point at which x is maximum is the answer because that is the point which will have the maximum value after applying the operations.