92. Reverse Linked List II

public static ListNode reverseBetween(ListNode head, int m, int n) {
if(head == null | m < 1 || n < 1 || m >= n ) {
return head;
}

int count = 0;
ListNode prep = null,pre = null, last = null,lastl = null, cur = head;
while(cur != null) {
count++;
if(count == m - 1) {
prep = cur;
}
if(count == m) {
pre = cur;
}
if(count == n) {
last = cur;
}
if(count == n+1) {
lastl = cur;
}
cur = cur.next;
}
if(last == null) {
return head;
}
ListNode  pret = pre, tcur = pre.next,tcnext = null;
pret.next = lastl;
while(tcur != lastl) {
tcnext = tcur.next;
tcur.next = pret;
pret = tcur;
tcur = tcnext;
}
if(prep == null) {
head = pret;
}else {
prep.next = pret;
}
return head;
}