This type of diagnostic error (the one in your submission) usually happens in my case whenever I try to access a non-allocated memory space (like trying to access an element not in an array), I can see a lower_bound in your code, have you checked for it's edge cases, like when it throws last?

PS — I am only a noob so take this with a grain of salt

You do binary search on a not sorted array.

Error: elements in iterator range [__first, __last) are not partitioned by 
the value __val.

Still getting WA for this, would really appreciate if someone could point out what is wrong?