如何在数组中找到重复元素？

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.

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
)

$lookup = array();
foreach($array as $v) {
    if (!isset($lookup[$v]))
        $lookup[$v] = false;
    else if ($lookup[$v] == false) {
        echo "Duplicate $v
";
        $lookup[$v] = true;
    }
}

Hope this will work

array_unique(array_diff($inputArray, array_unique($inputArray)));

There is a array_count_values() function provided by PHP itself.

It does more than what you need, but should be quite fast since it's compiled-in..

Then, of course, you need to filter out the result for keys which value is > 1.

EDIT

If you want a one-liner:

$a = array('a','b','c','a','a','b','d','e');
array_keys(array_filter(array_count_values($a), create_function('$x', 'return $x>1;')));
// array (0 => 'a', 1 => 'b');