遍历，一共有三种情况，
1. pre <= x <= current
2. 遍历到了head时，x>tail 或者 x<=head (不会等于tail）
3. 遍历回aNode或同值的Node，此时直接插到此Node的前面
public void insert(Node aNode, int x) {
ListNode last = aNode;
ListNode current = aNode.next;
while (current.val != aNode.val) {
if(last.val<=x && x<=current.val) {
insertbtw(last,current,x);
return;
}
else {
if (last.val>current.val && (last.val<x || x<=current.val)) {
insertbtw(last,current,x);
return;
}
}
last = current;
current = current.next;
}
if(!inserted) {
insertbtw(last,current,x);
}
return;
}
public void insertbtw(ListNode last, ListNode current, int x) {
ListNode temp = new ListNode(x);
last.next = temp;
temp.next = current;
return;
}