有没有复杂度为O(n)的二维数组合并去重算法

<code class="php">$a = [
    ['id'=>1],
    ['id'=>2],
    ['id'=>3],
    ['id'=>4],
    ['id'=>5],
    ['id'=>6]
];
$b = [
    ['id'=>5],
    ['id'=>6],
    ['id'=>7],
];
$c = [
    ['id'=>8],
    ['id'=>9]
];

function array_unique_merge() {
    $params = func_get_args();
    $result = [];
    $hashmap = [];
    $arr_count = count($params);
    for($i = 0; $i $val) {
            $md5 = md5(json_encode($val));
            if (!isset($hashmap[$md5])) {
                $hashmap[$md5] = true;
                $result[] = $val;
            }
        }
    }
    return $result;
}

print_r(array_unique_merge($a, $b, $c));</code>