Getting unexpected result in unordered map

You have two problems:

1. if(m[ele-1]) will insert a new (default-constructed) element with key ele-1 if it wasn't already there, as documented (and the ele+1 version does the same)

   This is a logic error, in that your program is doing something that, although well-defined, is not what you wanted. You should use find if you want to find out whether something exists without default-constructing it.

2. When you mutate the container (by inserting an element instead of finding it) - this may invalidate the iterators you're using to walk the container. Again, see the same documentation:

   If an insertion occurs and results in a rehashing of the container, all iterators are invalidated.

   This is a more serious error, because continuing to use those iterators at all is Undefined Behaviour. It's not a logic error, but a bug.

Reasonable code might look like

        for(auto it=m.begin(); it!=m.end(); it++)
        {
            cout << it->first <<" ";
            auto pred = m.find(it->first - 1);
            auto succ = m.find(it->first + 1);

            if(pred != m.end())
                maxLen=max(maxLen, it->second + pred->second);
            if(succ != m.end())
                maxLen=max(maxLen, it->second + succ->second);       
        }