Your example as you have literally explained it entails just deleting all repeats (ambiguous if 121 -> 12 or 121, or for that matter if 121 is possible as an input). Either way a simple for loop could solve it. But you are a self proclaimed smart person who eats for loops for breakfast, so it seems your have over simplified your description of the problem.

Machine learning is essentially the following:

• For sets X and Y, take a collection of some functions F = {f: X -> Y}
• Take a probability distribution D on X x Y. Note this could be of the form (x, g(x)) for fixed unknown function g where only x is random.
• Take a loss function L: F -> R, eg L(f) = ExpectedValue( distance(f(x),y) )
• Sample data from the distribution
• ???
• Have an f in F that is close to minimizing L

So tell us what F, X, Y, D, L are.