Order numbers

I've got a grid with an index on it, before saving the info i need to ensure that the numbers are all there from 1 to max and that there are no duplicates exist, e.g. 1,2,4,5 should change to 1,2,3,4

2,1,6,4 should be 2,1,4,3
9,4,9,1,6,8 should be 5,2,6,1,3,4

i was wondering how you guys would attempt to program this.