3009. Maximum Number of Intersections on the Chart

We move a horizontal line from the bottom of the chart to the top.

For each point we reach, the number of intersections might change, so we have to count it.

If a point we just reached is lower than its previous/next point, the number of intersections increases.

If a point we just reached is higher than its previous/next point, the number of intersections decreases.

There is also another solution using Fenwick Tree.