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Contents.swift
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Contents.swift
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import UIKit
/**
Reverse a singly linked list.
Example:
Input: 1->2->3->4->5->NULL
Output: 5->4->3->2->1->NULL
Follow up:
A linked list can be reversed either iteratively or recursively. Could you implement both?
*/
class ListNode: CustomStringConvertible {
var val: Int
var next: ListNode?
init(_ val: Int) {
self.val = val
self.next = nil
}
var description: String {
var str = ""
var node: ListNode? = self
var first: Bool = true
while node != nil {
str += first ? "\(node!.val)" : "->\(node!.val)"
node = node!.next
first = false
}
return str
}
}
/// Iteratively
func reverseListByIteratively(_ head: ListNode?) -> ListNode? {
var newHead: ListNode? = nil
var tempHead = head
while tempHead != nil {
let next = tempHead!.next
tempHead!.next = newHead
newHead = tempHead
tempHead = next
}
return newHead
}
/// Recursively
func reverseListByRecursively(_ head: ListNode?) -> ListNode? {
return reverseByRecursively(head: head, newHead: nil)
}
func reverseByRecursively(head: ListNode?, newHead: ListNode?) -> ListNode? {
if head == nil {
return newHead
}
let next = head?.next
head?.next = newHead
return reverseByRecursively(head: next, newHead: head)
}
var node1 = ListNode(1)
var node2 = ListNode(2)
var node3 = ListNode(3)
var node4 = ListNode(4)
var node5 = ListNode(5)
node1.next = node2
node2.next = node3
node3.next = node4
node4.next = node5
print(reverseListByRecursively(node1)?.description)