example: if i have 999 number combinations but im missing the 1000th number combo, how can i find the combo? is there some software that allows me too do this, im trying to find a number generator but in reverse!. like if you go on number generator sites they say put in your numbers e.g 1,2,3,15,20 then they give you a list of combinations but i want to go on a website/software and put in the list of combinations from the numbers (1,2,3,15,20) and then let the software come back and say your missing combo.. 15,2,1,20,3 etc

so if anyone knows any software that is capable of performing these actions?

or maybe links too some websites that has what im looking for, thanks.