Question

I have an array of arrays:

(1, 3), (3, 5, 7), (2, 3, 4), (3, 7, 9)

I need to find a value that appears in all sub-arrays,
and in this case that number is “3”.

The most efficient solution I could find was O(n^3) and I was wondering whether there is more efficient way to achieve this.

Answer 1

You can try with array_intersect:

$output = array_intersect($array1, $array2, $array3, ...);

or much automatically:

$input  = array( array(1, 3), array(3, 5, 7), array(2, 3, 4), array(3, 7, 9) );
$output = $input[0];

foreach ( $input as $key => $subarray ) {
  if ( $key == 0 ) {
    continue;
  }

  $output = array_intersect($output, $subarray);
  if ( empty($output) ) {
    break;
  }
}

var_dump($output);