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➤微信公众号：山青咏芝（shanqingyongzhi）
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Given a linked list, reverse the nodes of a linked list k at a time and return its modified list.
k is a positive integer and is less than or equal to the length of the linked list. If the number of nodes is not a multiple of k then left-out nodes in the end should remain as it is.
Example:
Given this linked list: 1->2->3->4->5
For k = 2, you should return: 2->1->4->3->5
For k = 3, you should return: 3->2->1->4->5
Note:
- Only constant extra memory is allowed.
- You may not alter the values in the list's nodes, only nodes itself may be changed.
给出一个链表，每 k 个节点一组进行翻转，并返回翻转后的链表。
k 是一个正整数，它的值小于或等于链表的长度。如果节点总数不是 k 的整数倍，那么将最后剩余节点保持原有顺序。
示例 :
给定这个链表：1->2->3->4->5
当 k = 2 时，应当返回: 2->1->4->3->5
当 k = 3 时，应当返回: 3->2->1->4->5
说明 :
- 你的算法只能使用常数的额外空间。
- 你不能只是单纯的改变节点内部的值，而是需要实际的进行节点交换。
48ms
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 reverseKGroup(_ head: ListNode?, _ k: Int) -> ListNode? { 14 let dummyHead = ListNode(0) 15 dummyHead.next = head 16 17 var previous: ListNode? = dummyHead 18 var first = head 19 var kthNode: ListNode? = first 20 for _ in 1..<k { 21 kthNode = kthNode?.next 22 } 23 24 while kthNode != nil { 25 26 let nextNode = kthNode?.next 27 28 kthNode?.next = nil 29 30 previous?.next = reverseList(head: first) 31 first?.next = nextNode 32 33 previous = first 34 first = previous?.next 35 kthNode = first 36 for _ in 1..<k { 37 kthNode = kthNode?.next 38 } 39 40 } 41 42 return dummyHead.next 43 } 44 45 func reverseList(head: ListNode?) -> ListNode? { 46 var oldList = head 47 var newList: ListNode? = nil 48 49 while let node = oldList { 50 let nextNode = node.next 51 node.next = newList 52 newList = node 53 54 oldList = nextNode 55 } 56 return newList 57 } 58 }
52ms
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 reverseKGroup(_ head: ListNode?, _ k: Int) -> ListNode? { 14 if head?.next == nil { 15 return head 16 } 17 let dummy: ListNode? = ListNode(0) 18 dummy!.next = head 19 var pre = dummy, start = dummy, end = dummy 20 while end != nil { 21 end = pre 22 for _ in 0..<k { 23 end = end?.next 24 if end == nil { 25 return dummy?.next 26 } 27 } 28 for _ in 1..<k { 29 start = pre!.next 30 pre!.next = start!.next 31 start!.next = end!.next 32 end!.next = start 33 } 34 for _ in 0..<k { 35 pre = pre!.next 36 } 37 } 38 return dummy!.next 39 } 40 }
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 reverseKGroup(_ head: ListNode?, _ k: Int) -> ListNode? { 14 let dummyHead = ListNode(0) 15 dummyHead.next = head 16 var previous = dummyHead 17 var start = dummyHead.next 18 var end = start 19 while end != nil { 20 for _ in 0..<k - 1 { 21 end = end == nil ? nil : end!.next 22 } 23 if end != nil { 24 let next = end!.next 25 var newStart = start 26 while start!.next !== next { 27 let nextNext = start!.next!.next 28 previous.next = start!.next 29 start!.next!.next = newStart 30 newStart = previous.next 31 start!.next = nextNext 32 } 33 previous = start! 34 start = next 35 end = start 36 } 37 } 38 return dummyHead.next 39 } 40 }
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 reverseKGroup(_ head: ListNode?, _ k: Int) -> ListNode? { 14 if k == 1 { 15 return head 16 } 17 18 var nextNode = head 19 var prevTail: ListNode? 20 var res: ListNode? 21 while nextNode != nil { 22 var newHead: ListNode? 23 var newTail: ListNode? 24 25 (newHead, nextNode, newTail) = reverseK(node: nextNode,parent: nil, k: k-1) 26 if res == nil { 27 res = newHead 28 } 29 prevTail?.next = newHead 30 prevTail = newTail 31 print(newHead?.val, nextNode?.val) 32 } 33 return res 34 35 } 36 37 func reverseK(node: ListNode?, parent: ListNode?, k: Int) -> (ListNode?, ListNode?, ListNode?) { 38 if k <= 0 { 39 let temp = node?.next 40 node?.next = parent 41 return (node, temp, nil) 42 } else if node == nil { 43 return (nil, nil, nil) 44 } 45 46 let (newHead, nextNode, _) = reverseK(node: node?.next, parent: node, k: k-1) 47 48 if newHead != nil { 49 var tail: ListNode? 50 if parent == nil { 51 node?.next = nextNode 52 tail = node 53 } else { 54 node?.next = parent 55 } 56 return (newHead, nextNode, tail) 57 } else { 58 if parent == nil { 59 return (node, nil, nil) 60 } else { 61 return (nil, nil, nil) 62 } 63 } 64 } 65 }
84ms
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 reverseKGroup(_ head: ListNode?, _ k: Int) -> ListNode? { 14 var head:ListNode? = head 15 var current_node:ListNode? = head 16 var count:Int = 0 17 while(current_node != nil && count != k) 18 { 19 current_node = current_node!.next 20 count++ 21 } 22 if count == k 23 { 24 // 递归的解决子问题 25 current_node = reverseKGroup(current_node, k) 26 while (count-- > 0) 27 { 28 var temp:ListNode? = head!.next 29 head!.next = current_node 30 current_node = head 31 head = temp 32 } 33 //最终，该段的所有节点将会截空，head应指向current_node 34 head = current_node 35 } 36 return head 37 } 38 } 39 40 /*扩展Int类，实现自增++、自减--运算符*/ 41 extension Int{ 42 //后缀++:先执行表达式后再自增 43 static postfix func ++(num:inout Int) -> Int { 44 //输入输出参数num 45 let temp = num 46 //num加1 47 num += 1 48 //返回加1前的数值 49 return temp 50 } 51 //后缀--:先执行表达式后再自减 52 static postfix func --(num:inout Int) -> Int { 53 //输入输出参数num 54 let temp = num 55 //num减1 56 num -= 1 57 //返回减1前的数值 58 return temp 59 } 60 }