PHP特定类型的数字

I have the following type of array :

$foo = array(
         "a" => 1,
         "b" => 1,
         "c" => 2,
         "d" => 2,
         "e" => 3,
         "f" => 3,
         "g" => 4,
         "h" => 4,
         "i" => 5,
         "j" => 5,
         "k" => 10,
         "l" => 12,
         "m" => 15,
         "n" => 20
       );

I need to sort the array in this way :

$foo = array(1,2,3,4,5,12,20,15,10,5,4,3,2,1);

As you can see, the best value needs to be in the middle of the array. And the min-values in the start/end of array. The key needs to be linked to the original value.

Thank's for the help! sorry for my English.

If the input is already sorted, you can use two loops to first push every item with an odd offset in ascending order and then every item with an even offset in descending order into your array:

$keys = array_keys($foo);
$n = count($keys);
$result = array();
for ($i=0; $i<$n; $i+=2) {
    $result[$keys[$i]] = $foo[$keys[$i]];
}
for ($i=$n-$n%2-1; $i>0; $i-=2) {
    $result[$keys[$i]] = $foo[$keys[$i]];
}

First arrange all the integers in ascending order. Lets say $foo_as is the new array you got in ascending order.

$a=a new empty array of size $n

$n=number of integers.
$c=0;
for ($i=$n-1;$i>0;$i--){
   if (i%2==0){
      $a(i/2)+c=$foo_as($n-$c)
   }
   else{
      $a(i+1/2)-c=$foo_as($n-$c)
   }
   $c++
}

Explanation: I'm putting the largest integer in the middle. After that I'm filling the alternate indexes around the middle one with the integers in descending order.

-   -   -   -   -   11  -   -   -   -   -
-   -   -   -   10  11  -   -   -   -   -
-   -   -   -   10  11  9   -   -   -   -
-   -   -   8   10  11  9   -   -   -   -
-   -   -   8   10  11  9   7   -   -   -
-   -   6   8   10  11  9   7   -   -   -
-   -   6   8   10  11  9   7   5   -   -
-   4   6   8   10  11  9   7   5   -   -
-   4   6   8   10  11  9   7   5   3   -
2   4   6   8   10  11  9   7   5   3   -
2   4   6   8   10  11  9   7   5   3   1

I have used a basic algorithm to demonstrate this. If at some point, my php syntax is incorrect, please omit that.

A overkill version:

<?php
$foo = array(
        "a" => 1,
        "b" => 1,
        "c" => 2,
        "d" => 2,
        "e" => 3,
        "f" => 3,
        "g" => 4,
        "h" => 4,
        "i" => 5,
        "j" => 5,
        "k" => 10,
        "l" => 12,
        "m" => 15,
        "n" => 20
);

$arrayKeys = array_keys($foo);
$arrayValues = array_values($foo);

$array_count = count($foo);
for ($idx =0; $idx < round($array_count/2); $idx+=2) {
    $tmpA = $arrayKeys[$idx];
    $arrayKeys[$idx] = $arrayKeys[$array_count - $idx -1];
    $arrayKeys[$array_count - $idx -1] = $tmpA;

    $tmpB = $arrayValues[$idx];
    $arrayValues[$idx] = $arrayValues[$array_count - $idx -1];
    $arrayValues[$array_count - $idx -1] = $tmpB;
}

$tmpArray = array_combine($arrayKeys, $arrayValues);

$ascent = array_slice($tmpArray, 0, round($array_count/2));
$descent = array_slice($tmpArray, round($array_count/2));

asort($ascent);
arsort($descent);

$foobar = array_merge($ascent, $descent);

var_dump($foo, $foobar);
?>