Merge and Sort in Array

Question

In this question I am trying to solve the question of merge and sort. I think my approach is correct but in the question its written that

You have to return the size of the array formed (4 in this case) and update the answer array in the function mergeNsort().

so I am unable to do its step.

class Solution{
    public:


    int mergeNsort(int a[], int b[], int n, int m, int answer[])
    {
       vector<int> z;
       int i=0;
       int j=0;
       while(i<n && j<m)
       {
           if(a[i]<b[j])
           {
               z.push_back(a[i]);
               i++;
           }
           else
           {
               z.push_back(a[j]);
               j++;
           }
       }
       while(i>n)
       {
           z.push_back(a[i]);
           i++;
       }
       
       while(j>m)
       {
           z.push_back(b[j]);
           j++;
       }
       
       for(int i=0;i<n;i++)
       {
           answer[i]=z[i];
       }
       
    }
};