Merge Sort Function not working when size of vector is not 2^n

I want to preface this question by saying I'm a beginner to c++.

For practice, I tried creating a merge sort function using the elementary knowledge that I have learnt. The code is as follows:

#include <bits/stdc++.h>
using namespace std;

vector<int> mergesort(vector <int> x);
int main(){
    vector<int> tosort={1,9,0};
    tosort=mergesort(tosort);
    for(int  i=0; i<tosort.size(); i++){
        cout<<tosort[i]<<" ";
    }

}
vector<int> mergesort(vector <int> x){
   if (x.size()==1){
       return x ;
   }
   else{
       int vsize=x.size();
       vector<int> halfv1(x.begin(),x.begin()+ceil(vsize / 2));
       vector<int> halfv2(x.begin()+ceil(vsize / 2),x.end());
       
       
       halfv1=mergesort(halfv1);
       halfv2=mergesort(halfv2);
       int counter1=0;
       int counter2=0;
       while(counter1+counter2<vsize){
           if(counter1==ceil(vsize / 2)){
               for(int i=counter1+counter2;i<vsize;i++){
                   x[i]=halfv2[counter2];
                   counter2++;
               }


           }
           if(counter2==floor(vsize / 2)){
               for(int i=counter1+counter2;i<vsize;i++){
                   x[i]=halfv1[counter1];
                   counter1++;
               }

           }
           if(counter1 !=ceil(vsize / 2) && counter2 !=floor(vsize / 2)){
               if(halfv1[counter1 ]<=halfv2[counter2]){
                   x[counter1+counter2]=halfv1[counter1];
                   counter1++;
               }
               else{
                   x[counter1+counter2]=halfv2[counter2];
                   counter2++;
               }
           }
       }
       return x;
   }
}

The code works fine when the size of the vector tosort is 2^n. However, if it isn't then the output is erroneous. For example, in the above example the output is 0 1 0. Please can you help me identify the issue.

I have tried working through the logic line by line and there doesn't seem to be a mistake. I suspect there is an issue with regards to knowledge that I'm not aware of.

Please can you help me out considering I'm a beginner. Also I know there are probably more efficient ways to write the code but I just wanted to practice my algorithm skills with my current knowledge and find my gap in my understanding here.

Thank you very much for your help.

The immediate issue is that the size of halfv2 is not floor(vsize / 2) but vsize - vsize / 2 (or (vsize + 1) / 2)), so you need to change all those comparisons to not access indexes out of bounds. (Note that ceil was only applied to the truncated result of vsize / 2.)

Consider comparing against the size() of the vectors instead.

In addition, std::merge provides the functionality you are trying to implement here.