Today I tried to solve 1932F - Feed Cats, and finally got Accepted, though not after some hard debugging. You can give a try at the problem first before reading this.

Here is my original submission with WA on test 3 268592520 and here is my AC submission 268597071.

My thought process went like this:

• I thought of a segment tree with sweepline style solution

• Though using segment tree with n=10^6 is very risky (high constant time)

• So I went ahead and compress the coordinates of the start point and end point of each segment.

• Query an addition at the compressed start point and a removal at the (compressed end point) +1.

This is WRONG, however. Since there is a chance of nothing happening at the compressed end point and something else happening at the (compressed end point) +1 that might influence the answer, this can give a wrong result. The really important part is the compressed (end point +1) where the removal ACTUALLY happens, which I failed to realise for 90 minutes.

Did I learn my lesson? Yes. Did you waste your time reading something you already know/ don't need to know yet? I hope not. Either way, perhaps I will leave this blog here as a cautionary tale for myself, to better tackle future problems I will have to solve.