conservation of linear momentum during collision

Let us consider two particles of masses m1 and m2 which are collided with each other with velocities u1 and u2 respectively. After collision, they get the velocities v1 and v2 respectively.
When there is no external force, the momenta of two particles p1 and p2 respectively and the total momentum of the system remains constant. i.e.,p=p1+p2
From above equation, we can say that algebraic sum of momenta of a system of particles before collision is equal to the algebraic sum of momenta of a system of particles after collision.