$locations = array(
    3 => array("Building", 2),
    2 => array("Area", 1),
    0 => array("Floor", 3),
    1 => array("City"),
    4 => array("Room", 0),

    13 => array("Building1", 12),
    12 => array("Area1", 11),
    14 => array("Room1", 10),
    10 => array("Floor1", 13),
    11 => array("City1")
);

$route = [];

foreach ($locations as $locationKey => $locationArray) {

    if (!isset($locationArray[1])) continue;

    $nextLocation = $locations[$locationArray[1]][0];
    $route[] = $nextLocation;
}