Question

I have this assignment to prove that this problem:

Finite alphabet £, two strings x,y €
£*, and a positive integer K. Is
there a way to derive the string y
from the string x by a sequence of K
or fewer operations of single symbol
deletion or adjacent symbol
interchange?

is np-complete. I already figured out I have to make transformation from decision version of set covering problem, but I have no clue how to do this. Any help would be appreciated.

Answer 1

It looks like modified Levenshtein distance. Problem can be solved with DP in quadratic time.

Transformation from minimum set cover (MSC) to this string correction problem is described in:

Robert A. Wagner
On the complexity of the Extended String-to-String Correction Problem
1975, Proceedings of seventh annual ACM symposium on Theory of computing 

In short with MSC problem:

Given finite sets x_1, …, x_n, and integer L, does there exists a subset J of {1,…,n} such that |J| <= L, and

union_{j in J} x_j = union all x_i ?

Let w = union all x_i, let t = |w| and r = t^2, and choose symbols Q, R, S not in w.

Take strings:

A = Q^r R x_1 Q^r S^(r+1) ... Q^r R x_n Q^r S^(r+1)
B = R Q^r ... R Q^r w S^(r+1) ... S^(r+1)   <- each ... is n times
and
k = (l+1)r - 1 + 2t(r+1)(n-1) + n(n-1)(r+1)^2/2 + (r*n + |x_1 ... x_n| - t)*W_d
[W_d is delete operation weight, can be 1.]

It is shown that string-string correction problems (A,B,k) is satisfiable iff source MSC problem is.

From strings construction it is clear that proof is not trivial 🙂 But it isn’t too complex to manage.