[Swift]LeetCode234. 回文链表 | 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?

---

 请判断一个链表是否为回文链表。

示例 1:

输入: 1->2
输出: false

示例 2:

输入: 1->2->2->1
输出: true

进阶：
你能否用 O(n) 时间复杂度和 O(1) 空间复杂度解决此题？

---

 72ms

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public var val: Int
 *     public var next: ListNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.next = nil
 *     }
 * }
 */
class Solution {
    func isPalindrome(_ head: ListNode?) -> Bool {
        var root = head
        var stack = [ListNode]()
        while root != nil {
            stack.append(root!)
            root = root?.next
        }
        
        var low = 0
        var high = stack.count - 1
        
        while low < high {
            let lownode = stack[low]
            let highnode = stack[high]
            
            if lownode.val == highnode.val {
                low += 1
                high -= 1
            } else {
                return false
            }
        }
        return true
    }
}

---

56ms

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public var val: Int
 *     public var next: ListNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.next = nil
 *     }
 * }
 */
class Solution {
    func isPalindrome(_ head: ListNode?) -> Bool {
        var slow = head
        var fast = head
        var reverse: ListNode? = nil
        
        while fast?.next != nil {
            fast = fast!.next!.next
            let next = slow!.next
            slow!.next = reverse
            reverse = slow
            slow = next
        }
        if fast != nil {
            slow = slow!.next
        }
        
        while slow != nil {
            if slow!.val != reverse!.val {
                return false
            }
            slow = slow!.next
            reverse = reverse!.next
        }
        
        return true
    }
}

---

60ms

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public var val: Int
 *     public var next: ListNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.next = nil
 *     }
 * }
 */
class Solution {
    func isPalindrome(_ head: ListNode?) -> Bool {
        var slow = head
        var fast = head
        var reverse: [Int] = [Int]()
        reverse.reserveCapacity(100)
        
        while fast?.next != nil {
            fast = fast!.next!.next
            reverse.append((slow?.val)!)
            slow = slow?.next
        }
        if fast != nil {
            slow = slow!.next
        }
        
        while slow != nil {
            if slow!.val != reverse.popLast() {
                return false
            }
            slow = slow!.next
        }
        
        return true
    }
}