Reverse Linked List II

public class Solution {
    public ListNode reverseBetween(ListNode head, int m, int n) {
        ListNode h = new ListNode(-1);
        h.next = head;
        ListNode pre = h;
        int cnt = 1;
        while(cnt<m){
             pre = pre.next;
             cnt++;
        }
        ListNode mnode = pre.next;
        ListNode nnode = mnode;
        while(cnt<n){
            nnode = nnode.next;
            cnt++;
        }
        ListNode end = nnode.next;
        reverse(pre, end);
        return h.next;
    }
    
    public void reverse(ListNode pre, ListNode end){ // B keep to being inserted after pre  pre-1-2-3-end, 1= A, 2=B, 3=t
        ListNode A = pre.next, B  = A.next; // A 定义为B前面一位, 不断的把B塞到pre后面（也就是list的第一位，然后把原来B的前面A链接到B的后面t上去
        while( B!=end){
            ListNode t =  B.next;
            B.next = pre.next;
            pre.next = B;
            A.next = t;
            B=t;
        }
    }
}