Re^2: Longest Common SubSequence Not Working Correctlyby blokhead (Monsignor)
|on Nov 13, 2007 at 20:01 UTC||Need Help??|
Don't let the name of the subroutine fool you. The "brute force" algorithm is not really "brute forcing" the problem. A brute force approach would be to consider every possible subsequence of the strings, taking O(2min(x,y)) time.
In fact, the "brute force" algorithm is doing the same thing as the recursive algorithm (i.e., doing the dynamic programming solution), but iteratively. It uses a standard trick for making a recursive memoizing dynamic programming algorithm iterative. Since the two algorithms solve the problem in essentially the same way, but the iterative one doesn't have the overhead of subroutine calls (which are slow in Perl), it is no surprise that the iterative one is faster.
Usually it's easier and more intuitive to write a dynamic programming problem in terms of recursive calls. However, it's necessary to memoize the result of each recursive call, because several other subproblems might use that result of this subproblem in their computation.
Now imagine a table that holds all of these memoized results. What happens to this table while the recursive algorithm is running? The table is gradually filling up. How does it fill up? Well, in this case, to compute the value of the subproblem ($a,$b), I need to get the solutiosn for at most these three subproblems:
($a,substr($b,0,-1)), (substr($a,0,-1),b), (substr($a,0,-1),substr($b,0,-1))In other words, I need to have those 3 cells in the table filled in before I can fill in this cell.
So suppose I now do things iteratively instead of recursively, and just concentrate on filling up the table. I'll visit the table's cells in such a way so that I visit the cell ($a,$b) after I visit the three above cells. That way, to fill up the cell ($a,$b), I just check those 3 other cells, do some local comparisons, and I'm done. Finally, the last cell in the table is generally the answer to the "main" subproblem, and I can return that. That's exactly what this "brute force" algorithm is doing.