题目
反转一个单链表。
示例:
输入:1->2->3->4->5->NULL
输出:5->4->3->2->1->NULL
进阶:
你可以迭代或递归地反转链表。你能否用两种方法解决这道题？
方案一：
- 迭代：通过while 迭代反转链表
/**
public class SingNode {
public var value : Int
public var nextNode: SingNode?
public init(value:Int) {
self.value = value
}
}
**/
func reverseList(_ head:SingNode?) -> SingNode? {
if head == nil || head?.nextNode == nil {
return head
}
var newNode:SingNode? = nil
var p = head
while p != nil {
let temp = p?.nextNode
p?.nextNode = newNode
newNode = p
p = temp
}
return newNode
}
方案二：
- 迭代： 迭代反转链表
/**
public class SingNode {
public var value : Int
public var nextNode: SingNode?
public init(value:Int) {
self.value = value
}
}
**/
func reverseList(_ head: SingNode?) -> SingNode? {
if head == nil || head?.next == nil {
return head
}
let newHead = reverseList(head?.next)
head?.next?.next = head
head?.next = nil
return newHead
}
