用bitmap实现中位数的算法[ZT]

用bitmap实现中位数的算法[ZT]

http://blog.csdn.net/hfahe/archive/2010/05/07/5567259.aspx

define("MASK", 0x1f); 

$source = array(1, 74, 4, 256, 1024, 110, 111, 112, 123, 112, 100); 

$array = array(); 

$count = 0; 

foreach($source as $num) { 

    set($num); // add to bit map

} 

$count = intval($count >> 1) + 1; // cal middle number

for($i = 0;;$i++) { // travel the bit map and find the middle number

$num = $array[$i]; 

if(!$num) { 

continue; 

    } 

$sum = 0; 

while($num) { 

if($num & 0x1) { 

if(!--$count) { 

echo ($i << 5) + $sum; 

exit;        

            } 

        } 

$num >>= 1; 

$sum++; 

    } 

} 

/*

* set number to bit map

*/

function set($i) { 

global $array; 

global $count; 

$temp = (1 << ($i & MASK)); 

$array[$i >> 5] |= $temp; 

if($temp) { 

$count++; 

    } 

}