Swift: How to compare Optional in "if" clause

I'm practising Swift by building trees with array.

I have a class Node. Each node has an ID and a parent ID. The parent ID could be nil, if the node is a top-level node. So I made parent ID Optional.

Then I manually (for testing purpose) created an array of nodes, which contains multiple trees. So there are multiple nodes with parent ID = nil.

Then I created a function which find nodes by their parent ID.

Compiler didn't complain. However when run in iPhone simulator (I tried to display the nodes in a tableview), a message displays at the if clause in the function:

fatal error: unexpectedly found nil while unwrapping an Optional value

Please help. Thanks.

Here is the code. (for my own habit I write ";" at the end of each line.)

class Node {
  var id:Int;
  var parent:Int!;
  init(id:Int, parent:Int!) {
    self.id = id;
    self.parent = parent;
  }
}

var allNodes = [
  Node(id:1, parent: nil),
  Node(id:2, parent: nil),
  Node(id:3, parent: 1),
  Node(id:4, parent: 1),
  Node(id:5, parent: 2),
  Node(id:6, parent: 2),
  Node(id:7, parent: 2)
];

func findNodes(parent:Int!) -> [Node] {
  var arr:[Node] = [];
  for node in allNodes {
    if node.parent == parent! {
      // if I use "if node.parent == parent" without the "!" it gives me the same error
      arr.append(node);
    }
  }
  return arr;
}

let nodes = self.findNodes(nil);
// called in tableview functions.
// same message if I use findNodes(1).