冒泡排序

两两相邻的数进行比较，如果反序就交换，否则不交换
时间负责度：O(n^2)
空间负责度：O(1)

<?php
$a = array(3,8,1,4,11,7);
$len = count($a);
for ($i=1; $i<$len; $i++) {
    for($j=$len-1; $j>=$i;$j--) {
        if ($a[$j] < $a[$j-1]){//如果改成从大到小的话，只要将if改成if ($a[$j] > $a[$j-1])
            $temp = $a[$j];
            $a[$j] = $a[$j-1];
            $a[$j-1] = $temp;
        }
    }
}
print_r($a);