题解 | #反转链表

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 
 * @return ListNode类
*/
func ReverseList( head *ListNode ) *ListNode {
    if head == nil || head.Next == nil {
        return head
    }
    next := head.Next
    newhead := ReverseList(next)
    next.Next =head
    head.Next = nil 
    return newhead
}


func ReverseListV1( head *ListNode ) *ListNode {

    var pre  *ListNode 
    cur := head

    for i := 0; ;i++ {
        if cur == nil {
            break
        }
        temp := cur.Next
        cur.Next = pre
        pre = cur
        cur  = temp 
    }
    return pre
}


func ReverseListV2( head *ListNode ) *ListNode {

    pre := &ListNode{}
    pre.Next = head

    for i := 0 ;; i++ {
        if head == nil || head.Next == nil {
            break
        }
        next := head.Next
        head.Next = next.Next
        next.Next = pre.Next
        pre.Next = next
    }

    return pre.Next
    
}