61. 旋转链表

题目链接：https://leetcode-cn.com/problems/rotate-list/
解法一和链表倒数第k个节点一样。

执行用时：12 ms, 在所有 Swift 提交中击败了78.65%的用户
内存消耗：13.6 MB, 在所有 Swift 提交中击败了17.98%的用户

class Solution {
    func rotateRight(_ head: ListNode?, _ k: Int) -> ListNode? {
        if head == nil {
            return nil
        }
        var slow = head, fast = head, len = 0
        var p = head
        while p != nil {
            p = p?.next
            len += 1
        }
        let step = k % len
        for i in 0 ..< step {
            fast = fast?.next
        }
        while fast?.next != nil {
            slow = slow?.next
            fast = fast?.next
        }
        fast?.next = head
        p = slow?.next
        slow?.next = nil
        return p
    }
}

解法二是将尾指针指向了头节点，形成了环。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public var val: Int
 *     public var next: ListNode?
 *     public init() { self.val = 0; self.next = nil; }
 *     public init(_ val: Int) { self.val = val; self.next = nil; }
 *     public init(_ val: Int, _ next: ListNode?) { self.val = val; self.next = next; }
 * }
 */
class Solution {
    func rotateRight(_ head: ListNode?, _ k: Int) -> ListNode? {
        if head == nil {
            return nil
        }
        var len = 1
        var p = head
        while p?.next != nil {
            p = p?.next
            len += 1
        }
        let step = len - k % len
        p?.next = head
        var newHead = head
        for i in 0 ..< step {
            p = p?.next
            newHead = newHead?.next
        }
        p?.next = nil
        return newHead
    }
}