CODEWITHSUNDEEP
swift
WishIHadThreeGuns
WishIHadThreeGuns

Reputation: 1479

Remove Element in Swift linked list

For a Node class

public class ListNode: CustomStringConvertible {
    public var description: String {
        return val.description + (next?.description ?? "")
    }
         public var val: Int
         public var next: ListNode?
         public init(_ val: Int) {
                 self.val = val
                     self.next = nil
             }
     }

with a head and nodes such that

let head = ListNode(4)
let second = ListNode(5)
let third = ListNode(1)
let forth = ListNode(8)
head.next = second
second.next = third
third.next = forth
forth.next = nil

We can remove the second node simply with 

head.next = third

Will second be deallocated and the memory freed up?

Or do we need to remove the reference from second such that

second.next = nil

Upvotes: 1

Views: 471

Answers (1)

Sweeper
Sweeper

Reputation: 273465

To actually observe the deinitialisations, we can add a deinit block to ListNode:

deinit {
    print("\(val) deinit")
}

Your current code will not cause anything to be deinitialised, because recall that objects are only deinitialised when no other object holds a reference to it. In your current code, the global scope still has the let constant second that holds a reference to the second node, even after head.next = third is executed. This causes the second node to not be deinitialised.

One way to cause the second node to deinitialise, then, is to remove all the let constants except head:

let head = ListNode(4)
head.next = ListNode(5) // second
head.next?.next = ListNode(1) // third
head.next?.next?.next = ListNode(8) // fourth

And then removing the second node:

head.next = head.next?.next

This causes the following to be printed:

5 deinit

indicating that the second node is deinitialised.

Setting second.next to nil is irrelevant here, as second.next holds a reference to the third node. Setting it to nil only decreases the number of references pointing to the third node, making an effort toward deinitialising the third node.

Upvotes: 1

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