gridland | Gridland - A Weird Little Time Waster by doublespeak games

I'm facing a weird overflow issue with a Hackerrank Problem. Here's the brief of the problem statement.

There is a 2D array, and in each row of the array there are lines starting at one cell and ending at another cell (left to right in the same row). Now there can be multiple lines in one row and they can certainly overlap one another. The task is to find the number of unoccupied cells in the 2D array.

It is important to note that the dimensions of the array can be really big upto 10^9. So this was my approach

• Use a map that would have the row number as the key and a vector of pairs as its corresponding value. The pairs are nothing but the start and end index of a line in that row. row -> vector>
• Sort each such vector by the first element of the pair (i.e the start index of the interval).
• Merge the overlapping intervals for all the rows. For each merged interval calculate its length wR - wL + 1 where wL and wR are the start and end indexes of the interval respectively. Add the length of all these intervals and store in a variable sum
• Lastly, subtract that sum from n*m to get the number of unoccupied cells.

Here is the relevant code