I implemented merge sort in c++

I implemented Merge Sort in C++ code: vector < int > merge ( vector < int > &p, vector < int > &q){     vector < int > ans;     int pi = 0 ,qj = 0 ;     while (pi < p. size () && qj < q. size ()) {         if (p[pi] <= q[qj]) {             ans. push_back (p[pi]);             pi++;         } else {             ans. push_back (q[qj]);             qj++;         }     }     if (pi == p. size ()) {         while (qj < q. size ()) {             ans. push_back (q[qj]);             qj++;         }     }     else {         ans. push_back (p[pi]);   ...