Convert function from recursion to iteration

问题:

I have this function that I wrote that is abysmally slow since php does not handle recursion well. I am trying to convert it to a while loop, but am having trouble wrapping my head around how to do it.

Can anyone give me some tips?

    public function findRoute($curLoc, $distanceSoFar, $expectedValue) {

    $this->locationsVisited[$curLoc] = true;
    $expectedValue += $this->locationsArray[$curLoc]*$distanceSoFar;

    $at_end = true;
    for($i = 1; $i < $this->numLocations; $i++) {
        if($this->locationsVisited[$i] == false) {
            $at_end = false;

            if($expectedValue < $this->bestEV)
                $this->findRoute($i, $distanceSoFar + $this->distanceArray[$curLoc][$i], $expectedValue);
        }
    }

    $this->locationsVisited[$curLoc] = false;

    if($at_end) {
        if($expectedValue < $this->bestEV) {
            $this->bestEV = $expectedValue;
        }
    }
}

回答1:

I'm not going to convert your code, but you can convert a recusive function into an iterative one by creating a stack:

$stack= array();

And instead of invoking $this->findroute(), push your parameters onto this stack:

$stack[] = array($i, $distanceSoFar + $this->distanceArray[$curLoc][$i], $expectedValue);

Now surround basically everything in your function into a while loop draining the stack after having primed it:

while ($stack) { 
    // Do stuff you already do in your function here

回答2:

You can convert a recursive function into an iterative function by using a stack to store the current state. Look into array_push() and array_pop().

回答3:

At a glance I don't think recursion is your problem, yes it's slow in PHP, but It looks like your going over values more than you need to, putting the values in a stack, or several stacks and handling them, may be nice.

custom sort functions have always helped me with problems of this sort.

function sort_by_visited($x,$y)
{
   return ($this->locationsVisited[$x] > $this->locationsVisited[$y]) ? -1 : 1;
}

uasort($locationsVisited,'sort_by_visited');

This will prioritize all not visited locations at the top of the stack.

回答4:

This looks like your trying to find the optimal route for traversal of a series of nodes in a graph.

I'm guessing that you've not studied Computer Science as the "Travelling Salesman" problem is an archetype of Artificial Intelligence. Of course, as such, it has its own Wikipedia page:

http://en.wikipedia.org/wiki/Travelling_salesman_problem

Sorry - but just swapping from a recursive to an iterative function isn't going to make it go any faster ("php does not handle recursion well." - can you provide reference for this assertion). If you need a faster solution then you'll need to look at non-exhaustive/fuzzy methods.

C.