php - Reverse string without strrev

Question

Some time ago during a job interview I got the task to reverse a string in PHP without using strrev.

My first solution was something like this:

$s = 'abcdefg';
$temp = '';
for ($i = 0, $length = mb_strlen($s); $i < $length; $i++) {
    $temp .= $s{$length - $i - 1};
}
var_dump($temp);
// outputs string(7) "gfedcba"

then they asked me if I could do this without doubling the memory usage (not using the $temp variable or any variable to copy the reversed string to) and I failed. This kept bugging me and since then I tried to solve this multiple times but I constantly failed.

My latest try looks like this:

$s = 'abcdefg';
for ($i = 0, $length = mb_strlen($s); $i < $length; $i++) {
    $s = $s{$i * 2} . $s;
}
var_dump($s);
// outputs string(14) "gfedcbaabcdefg"

It's not a solution to chop off "abcdefg" after the loop because then I would still double the amount of memory used. I need to remove the last character in every iteration of the loop.

I tried to use mb_substr like this:

$s = 'abcdefg';
for ($i = 0, $length = mb_strlen($s); $i < $length; $i++) {
    $s = $s{$i * 2} . mb_substr($s, $length - $i - 1, 1);
}
var_dump($s);

but it only gives me Uninitialized string offset errors.

This is where I'm stuck (again). I tried googling but all the solutions I found either echo the characters directly or use a temporary variable.

I also found the Question PHP String reversal without using extra memory but there's no answer that fits my needs.

Answer 1

That's an interesting one. Here's something I just came up with:

$s = 'abcdefghijklm';
for($i=strlen($s)-1, $j=0; $j<$i; $i--, $j++) {
    list($s[$j], $s[$i]) = array($s[$i], $s[$j]);
}
echo $s;

list() can be used to assign a list of variables in one operation. So what I am doing is simply swapping characters (starting with first and last, then second-first and second-last and so on, till it reaches the middle of the string)

Output is mlkjihgfedcba. Not using any other variables than $s and the counters, so I hope that fits your criteria.