This is what happened:

I was working on a WQS-Binary Search(Alien's Trick) problem today: CF125E. It's a template with many details; I wrote a bit over 100 lines of code. However, a few Runtime Errors (RE) submissions piqued my interest.

First was this RE on test 11: link. It was submitted using C++23. Then, without changing the code itself, I just switched the language to C++17, and it became WA (Wrong Answer) on test 14. The Checker Log showed Exit code is -1073741819.

After some debugging, it became RE on test 18: link. After some investigation, I suspected the issue might be with my sorting function. I had two sorting functions. The first sorted edges by weight ascending, and if weights were equal, edges connected to node 1 would be placed first. The second function reversed that priority for equal weights.

bool cmp1(edge x,edge y){
    if(x.w==y.w) return (x.u==1 || x.v==1);
    return x.w < y.w; 
}
bool cmp2(edge x,edge y){
    if(x.w==y.w) return (y.u==1 || y.v==1);
    return x.w < y.w;
}

I recalled encountering mysterious REs twice before on a similar problem on Luogu: my submission records, scoring 30 and 40 points respectively. Back then, modifying the sort function also fixed it.

So, I made a change: I manually recorded whether an edge was connected to node 1 within the struct, like this:

struct edge{
    int u, v, w, id;
    bool is1;
};
bool cmp1(edge x, edge y){
    if(x.w == y.w) return x.is1 > y.is1;
    return x.w < y.w; 
}
bool cmp2(edge x, edge y){
    if(x.w == y.w) return x.is1 < y.is1;
    return x.w < y.w;
}

Submitting this with C++20 worked and passed. It also passed with C++17.

I feel this is an important issue for me since it's happened two days in a row. I urgently want to understand the root cause of the REs to prevent similar problems in the future. Thank you.

Translated by Deepseek.