How to reverse the Linked List in Swift extension? - swift

Question

I want to reverse the Single Linked List in extension, but finally it's failed.can someone help me? thanks.
public class List<T: Equatable> {
var value: T!
var nextItem: List<T>?
public convenience init!(_ values: T...) {
self.init(Array(values))
}
init!(var _ values: Array<T>) {
if values.count == 0 {
return nil
}
value = values.removeFirst()
nextItem = List(values)
}
}
// Reverse a linked list.
extension List {
func reverse() {
}
}

Answer 1

I have a solution here. Please read the comments, its all evident from the code too. Also I have added a 'description' via CustomStringConvertible to the list to help debugging and printing.
public class List<T: Equatable>{
var value: T!
var nextItem: List<T>?
public convenience init!(_ values: T...) {
self.init(Array(values))
}
init!(_ values: Array<T>) {
if values.count == 0 {
return nil
}
var valuesCopy = values
value = valuesCopy.removeFirst()
nextItem = List(Array(valuesCopy))
}
}
extension List : CustomStringConvertible{
public var description: String {
var desc = String()
var listRef : List<T>? = self
while listRef != nil {
desc += "\((listRef?.value)!) "
listRef = listRef?.nextItem
}
return desc
}
}
extension List{
func reverse() -> List?{
// Iterate through each item, and reverse its link until you visit the last node in the list.
// Once you reach the end, All items except the last one would have
// their links reversed.
var nextNode : List<T>? = self.nextItem
var prevNode : List<T>? = nil
var currentNode : List<T>? = self
while nextNode != nil{
currentNode?.nextItem = prevNode
prevNode = currentNode
currentNode = nextNode
nextNode = currentNode?.nextItem
}
//Ensure the last item in the list too has its links reversed.
currentNode?.nextItem = prevNode
return currentNode
}
}
var list = List(1,2,3,5)
print(list ?? "Failed to create the list")
if let reversed = list.reverse(){
print(reversed)
}