Question

Let’s say I have an array of n elements(numbers or words), I want to find all the elements which occur more than once in the array. What is the most efficient approach of doing this in terms of performance?

PS: I can sort the array first, but only if that doesn’t affect the total performance much.
Also, though this is mainly php, i would like to know a algorithm or logic that can be implemented on other platforms too.

Answer 1

You can use array_count_values and array_filter

$array = array(1, "hello", 1, "world", "hello");
$new=array_filter(array_count_values($array),'custom_filter');
print_r($new);
function custom_filter($val)
{
  return $val > 1; 
}

output

Array
(
    [1] => 2
    [hello] => 2
)