Help regarding first subtask of BREAK problem

In the BREAK problem of MARCH LONG CHALLENGE 2020 , the first sub task gave WA in one of its test cases if i did it using a set but gave AC when i used map instead of set with the same logic. Can anyone identify the problem in the implementation using set , as it gave WA only in a single test case.
Link of the problem:

My solution using set:

My solution using map:

There’s quite a few differences between the two, but this at line 40 in your WA version leaps out:

           auto x=st.end();
            if(*it==st.size() && arr[i]!=*x)

st.end() is a one-past-the-end iterator, and dereferencing it is Undefined Behaviour:

Error: attempt to dereference a past-the-end iterator.

Objects involved in the operation:
    iterator "this" @ 0x0x7ffc418971d0 {
      type = __gnu_debug::_Safe_iterator<std::_Rb_tree_const_iterator<int>, std::__debug::set<int, std::less<int>, std::allocator<int> > > (mutable iterator);
      state = past-the-end;
      references sequence with type 'std::__debug::set<int, std::less<int>, std::allocator<int> >' @ 0x0x7ffc41897290
Aborted (core dumped)

I think you can use st.rend() if I’m not mistaken

1 Like

st.rbegin()” - “st.rend()” is also not de-referenceable :slight_smile:

There are other bugs, though - it is de-referenced without checking if it is valid; etc.


but why does it fail on only one test case?