In a university every student is given a student ID and university enrolment number. These both contain digits $(0-9).$ The student ID contains M digits. and the university enrolment number contains N digits. Every subject is given a unique subject code. While developing a student examination hall entrance system, the system must take the student ID$,$ the university enrolment number, and the subject code P for which the student is taking an examination. The system must generate the entrance code for the student from the digits of the student ID and the university enrolment number.

The generated entrance code must be the largest number of length P made up of digits from the student ID and the university enrolment number.

The relative order of the digits from the student ID and the university enrolment number must be preserved.

Write an algorithm to find the largest number of length P made up of digits from the student ID and the university enrolment number.