public void insertLast(ListNode x) {
if (front == null) {
front = back = x;
size++;
} else {
front.setNext(x);
front = x;
size++;
}
}
public void insertFirst(ListNode x) {
if (back == null) {
front = back = temp = x;
size++;
} else {
x.setNext(back);
back = x;
size++;
}
}