92. 反转链表 II

var reverseBetween = function(head, left, right) {
    if(!head)return null;
    let ret=new ListNode(-1,head),pre=ret,cnt=right-left+1;
    while(--left){
        pre=pre.next;
    }
    pre.next=reverse(pre.next,cnt);
    return ret.next;
}
var reverse=function(head,n){
    let pre=null,cur=head;
    while(n--){
        [cur.next,pre,cur]=[pre,cur,cur.next]
    }
    head.next=cur;
    return pre;
}