1) exactly ! I only cannot skip negative's or 0's if i am on corners

2) matrix is static fully known in advance (real matrix is 50000*50000 32bit int) so any preprocessing is more than welcomed
problem is that every time i get two different random values for which i need to compute the best solution. Optimal solution would be ok if it is within some proven cutoff (bound) from the best one (I would need to provide some guarantees if deciding to go for a suboptimal solution)

I'll check into the stated potions and repost my solution so maybe someone can do something better

Thank you !!!