swift - Extracting values from a reversed linked list -

question:

you have 2 numbers represented linked list, each node contains single digit. digits stored in reverse order, such 1 's digit @ head of list. write function adds 2 numbers , returns sum linked list.

an example:

input: (7-> 1 -> 6) + (5 -> 9 -> 2).

that is: 617 + 295.

output: 2 -> 1 -> 9.

that is: 912.

in order begin question, first created class define linked list:

step 1: defining linked list

class node: customstringconvertible{     var value: int     var next: node?     var description: string{         if next != nil {           return "\(value) -> \(next!)"         }         else{             return "\(value) -> \(next)"         }     }     init(value: int) {         self.value = value     } } 

step: 2 - generated linked list, user input of integer values

func generatelist (num: int) -> node {     var stringnum = array(string(num).characters)     let head = node.init(value:int(string(stringnum.first!))!)     var current = head      in 1..<stringnum.count{         let num = int(string(stringnum[i]))         current.next = node.init(value: num!)         current = current.next!     }     return head }  let list = generatelist(num: 716)  // following prints out: 7 -> 1 -> 6 -> nil 

then proceeded on reverse linked list using following function.

step 3: reverse linked list

func reverselinkedlist (head: node?) -> node?{      var current = head     var prev: node?     var next: node?      while current != nil {         next = current?.next         current?.next = prev         prev = current         current = next     }     return prev }  let reversedlist = reverselinkedlist(head: list)  // following prints out is: 6 -> 1 -> 7 -> nil 

step 4: idea behind step extract values on each of nodes, cast them string , concatenate them string variable , lastly cast string value int , use int value , add them.

func getvaluesfrom (head: node?) -> int {      var string = ""     var current = head      while current != nil {         var stringval = string(describing: current?.value)         string += stringval         current = current?.next     }      return int(string)! } 

here having problem:

when plug in following function so:

getvaluesfrom(head: reversedlist) 

i following error:

fatal error: unexpectedly found nil while unwrapping optional value

and can't seem figure out why having problem , appreciate sort of insight.

there no need convert , forth between string , linked list, except print results. done this:

class node {   var value: int   var next: node?    // init , generator can same method   init(value: int) {     // store ones place , divide 10     self.value = value % 10     var nextvalue = value / 10      // set loop     var currentnode = self      while nextvalue > 0 {       // create new node       // store ones place , divide 10       let next = node(value: nextvalue % 10)       nextvalue /= 10        // make new node next node       currentnode.next = next        // set next iteration       currentnode = next     }   } }  // make list printable extension node: customstringconvertible {   var description: string{     if let next = next {       return "\(value) -> \(next)"     }     else {       return "\(value) -> nil"     }   } } 

now can do:

print(node(value: 671)) // prints "7 -> 1 -> 6 -> nil" 

there no need reverse lists, given question.

to sum lists can you've said, convert int, add them, generate new list:

extension node {   func tovalue() -> int {     var place = 10     var current = self     // add each value , multiply place value     // first 1, second 10, third 100, etc.     var result = current.value     while let next = current.next {       result += next.value * place       place *= 10       current = next     }     return result   } } 

then need overload addition operator...

func +(lhs: node, rhs: node) -> node {   return node(value: lhs.tovalue() + rhs.tovalue()) } 

and test...

let first = node(value: 617) let second = node(value: 295) print(first) print(second) print(first + second) 

result:

7 -> 1 -> 6 -> nil

5 -> 9 -> 2 -> nil

2 -> 1 -> 9 -> nil