Can the for loop be eliminated from this piece of PHP code?

Question

I have a range of whole numbers that might or might not have some numbers missing. Is it possible to find the smallest missing number without using a loop structure? If ther

Answer 1

Technically, you can't really do without the loop (unless you only want to know if there's a missing number). However, you can accomplish this without first sorting the array.

The following algorithm uses O(n) time with O(n) space:

$range = [0, 1, 2, 3, 4, 6, 7];

$N = count($range);
$temp = str_repeat('0', $N); // assume all values are out of place

foreach ($range as $value) {
    if ($value < $N) {
        $temp[$value] = 1; // value is in the right place
    }
}

// count number of leading ones
echo strspn($temp, '1'), PHP_EOL;

It builds an ordered identity map of N entries, marking each value against its position as "1"; in the end all entries must be "1", and the first "0" entry is the smallest value that's missing.

Btw, I'm using a temporary string instead of an array to reduce physical memory requirements.