题解 | #牛群的重新分组#

package main

import (
	
	. "nc_tools"
)

/*
 * type ListNode struct{
 *   Val int
 *   Next *ListNode
 * }
 */

/**
 * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
 *
 *
 * @param head ListNode类
 * @param k int整型
 * @return ListNode类
 */


func reverse(head *ListNode) *ListNode {
    var prev *ListNode
    curr := head
    for curr != nil {
        nextTemp := curr.Next
        curr.Next = prev
        prev = curr
        curr = nextTemp
    }
    return prev
}
 
func reverseKGroup(head *ListNode, k int) *ListNode {
    if head == nil || k <= 1 {
        return head
    }
 
    dummy := &ListNode{Val:0, Next:head}
    prev := dummy
    for head != nil {
        tail := prev
        for i := 0; i < k; i++ {
            tail = tail.Next
            if tail == nil {
                return dummy.Next
            }
        }
        
        next := tail.Next
        
        tail.Next = nil
        prev.Next, head = reverse(head), next
        for i := 0; i < k; i++ {
            prev = prev.Next
        }
        prev.Next = next
    }
    return dummy.Next
}