[Swift]LeetCode234. 迴文鏈表 | Palindrome Linked List

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Given a singly linked list, determine if it is a palindrome.

Example 1:

Input: 1->2
Output: false

Example 2:

Input: 1->2->2->1
Output: true

Follow up:
Could you do it in O(n) time and O(1) space?

---

 請判斷一個鏈表是否爲迴文鏈表。

示例 1:

輸入: 1->2
輸出: false

示例 2:

輸入: 1->2->2->1
輸出: true

進階：
你可否用 O(n) 時間複雜度和 O(1) 空間複雜度解決此題？

---

 72ms

 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     public var val: Int
 5  *     public var next: ListNode?
 6  *     public init(_ val: Int) {
 7  *         self.val = val
 8  *         self.next = nil
 9  *     }
10  * }
11  */
12 class Solution {
13     func isPalindrome(_ head: ListNode?) -> Bool {
14         var root = head
15         var stack = [ListNode]()
16         while root != nil {
17             stack.append(root!)
18             root = root?.next
19         }
20         
21         var low = 0
22         var high = stack.count - 1
23         
24         while low < high {
25             let lownode = stack[low]
26             let highnode = stack[high]
27             
28             if lownode.val == highnode.val {
29                 low += 1
30                 high -= 1
31             } else {
32                 return false
33             }
34         }
35         return true
36     }
37 }

---

56ms

 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     public var val: Int
 5  *     public var next: ListNode?
 6  *     public init(_ val: Int) {
 7  *         self.val = val
 8  *         self.next = nil
 9  *     }
10  * }
11  */
12 class Solution {
13     func isPalindrome(_ head: ListNode?) -> Bool {
14         var slow = head
15         var fast = head
16         var reverse: ListNode? = nil
17         
18         while fast?.next != nil {
19             fast = fast!.next!.next
20             let next = slow!.next
21             slow!.next = reverse
22             reverse = slow
23             slow = next
24         }
25         if fast != nil {
26             slow = slow!.next
27         }
28         
29         while slow != nil {
30             if slow!.val != reverse!.val {
31                 return false
32             }
33             slow = slow!.next
34             reverse = reverse!.next
35         }
36         
37         return true
38     }
39 }

---

60ms

 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     public var val: Int
 5  *     public var next: ListNode?
 6  *     public init(_ val: Int) {
 7  *         self.val = val
 8  *         self.next = nil
 9  *     }
10  * }
11  */
12 class Solution {
13     func isPalindrome(_ head: ListNode?) -> Bool {
14         var slow = head
15         var fast = head
16         var reverse: [Int] = [Int]()
17         reverse.reserveCapacity(100)
18         
19         while fast?.next != nil {
20             fast = fast!.next!.next
21             reverse.append((slow?.val)!)
22             slow = slow?.next
23         }
24         if fast != nil {
25             slow = slow!.next
26         }
27         
28         while slow != nil {
29             if slow!.val != reverse.popLast() {
30                 return false
31             }
32             slow = slow!.next
33         }
34         
35         return true
36     }
37 }