I was solving question D of the latest div 2 contest. My idea is to pick the highest degree node at each step and set it first. I Know my first submission is a total mess, but it runs fine on my system, while it gives a runtime error when submitted. Can someone explain why this happens? Thanks! ps: I optimized the solution and got it accepted finally!

having an array $$$arr$$$ with $$$n$$$ elements we have two types of queries:

1. given an index $$$idx$$$ and a value $$$val$$$, $$$arr_{idx}:=val$$$
2. given a range $$$(l,r)$$$, find the $$$MEX$$$ of the range

what is the most efficient way to do this?

i'd be grateful if you could share some c++ code to do this.

thanks in advance guys.